Why are "Tax collectors and Sinners" usually mentioned together by the Pharisees?

Question

If sinners refers to the hard core, notoriously evil people (1 Timothy 1:9), why are tax collectors singled out? (When Jesus is caught eating with them, Luke 15:1, Mark 2:15, Matthew 9:11, 11:19, Luke 5:30) (Gk. 'oi 'amartoloi)

Or is sinners just a designation for ethnic (Gentile) peoples outside of Jewry? (With Gentiles being considered 'unclean', whether guilty of any sins or not. Galatians 2:15)

{And aside, did Jesus eat at their homes so often because they were the only sinners who had enough money to feed Him and all the Twelve?}

Were the tax collectors mentioned specifically because of the money involved, and the Pharisees were given to greed?

Answer 1

Here are some examples of "tax collectors and sinners". The last example is a bit different but still the same idea.

Matthew:5:46
If you love those who love you, what reward will you get? Are not even the tax collectors doing that?

Matthew 9:10
While Jesus was having dinner at Matthew’s house, many tax collectors and sinners came and ate with him and his disciples.

Matthew 21:32
For John came to you to show you the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes did. And even after you saw this, you did not repent and believe him.

Tax collectors in Jesus' day were not gentiles. They were Jews who were colluding with the Roman government to collect taxes from their own people. This job was bid out by the Roman government with the winning of the position going to the person who offered to collect the most taxes for a designated area.
They were viewed as traitors to God and to their country. Sinners, general sinners (also in this case Jewish) were better than tax collectors.

They are listed as "tax collectors and sinners" because tax collectors were seen as worse than regular sinners.

Why Exactly Were Tax Collectors so Hated?"

Jesus ate with tax collectors and sinners because they believed in him while many of the religious leaders did not - Matthew 21:32

Answer 2

Sinners

"Sinners" refers to Jews who did not follow the Law of Moses well. No one followed it perfectly, but "sinners" would be those who did not even attempt to adhere to it. The Gospel writers portray them not as evil people per se. We don't really know what percentage of the populace followed the law strictly. Indeed, one of the missions of the Pharisees was to educate people to do exactly that, to realize the biblical ideal of "a kingdom of priests and a holy nation." To quote Britannica:

The Pharisees tried to democratize the Jewish religion and remove it from the control of the Temple priests. The Pharisees asserted that God could and should be worshipped even away from the Temple and outside Jerusalem. To the Pharisees, worship consisted not in bloody sacrifices—the practice of the Temple priests—but in prayer and in the study of God’s law. Hence, the Pharisees fostered the synagogue as an institution of religious worship, outside and separate from the Temple.

Gentiles

In speaking of "sinners," the Gospels do not refer to Gentiles. Gentiles are a separate category, to whom most of the Mosaic laws do not apply. The Pharisees had diverse attitudes toward Gentiles. Jesus indicated that some Pharisees hoped to teach God's laws to foreigners. (Matthew 23:15: “You traverse sea and land to make one convert.") Other Pharisees refused to engage with Gentiles. The Talmudic story of Hillel and the Gentile exemplifies the attitudes of the two main branches of Pharisaism on this issue. (Hillel was the grandfather of the famous Gamaliel of Acts 5, incidentally.) Nor is there much evidence that the Pharisees were given to greed, other than the statement that they, as teachers of the Law, sought places of honor in synagogues and banquets (Matthew 23:6) The accusation of greed is more apt when applied to Sadducees, who were notorious collaborators with Rome in order to gain wealth and power.

Tax Collectors

As for tax collectors, they were indeed detested by Pharisees and the common people alike, because they served as Roman agents in a way which touched the people directly, even more than the Sadducees did by controlling the Temple and the high priesthood.

The term "tax collectors and sinners" is found seven times in the synoptic gospels and zero times in the Gospel of John. Since the synoptics borrow from one another freely, the frequency of the phrase is not as great as the OP suggests. Moreover, in the Gospel of Luke, the term "sinners" is used more often alone than in conjunction with tax collectors. In virtually all cases, "tax collectors and sinners" is used in order to emphasize Jesus' mission: “Those who are healthy do not need a physician, but the sick do." (Luke 5:31 etc.) In that sense, Jesus practiced what the Pharisees preached: to bring the masses to God way.

Answer 3

Tax-collectors in Israel are typically the Levites. All Levites residing outside of Jerusalem make a living from the tithes owed by people who can not travel to Jerusalem to pay their due. These Levites are then obliged to transfer a tithe of their income to the temple. Those Levites who fail to do so are of-course sinners and that is probably meant in Mark 2:15 as the Levite who hosted the party was obviously a corrupt official who kept the tithe due to the temple to himself to organize feasts with his other corrupt colleagues, the Pharisees are aware of this and speak out, but the reference that they themselves are going hungry are a tell to their own jealousy and indignation because they are not invited.