There are a couple different questions here. Let's unpack:
Why is the lab leak theory such a big deal?
Accountability, Reputation, Clicks, and Partisanship
Accountability - If the Covid crisis can be blamed on someone, especially if that someone (a government) is one of the richest entities in the world, it becomes a question of massive amounts of money and influence.
Reputation - If there was a leak that provided the responsible government with advance warning and information, that they did not share and instead covered up, that massively lowers the credibility of said government in any negotiation going forward, which makes this a question of money and influence.
Clicks - Conspiracy theories generate clicks and ad revenue. Conspiracy theories that slowly turn out to be more and more credible even more so.
Partisanship - When the theory started out, no proof was presented. It was an accusation without evidence, which at one point was promoted by a highly controversial source, Trump. People who are generally supportive of Donald Trump tend to find whatever he says to be credible, while people who are generally opposed to Donald Trump claim he has a long history of making false accusations without evidence to distract from his own shortcomings. This turns any new evidence that supports the theory into fuel for more partisan articles.
why would so much of the media and so many politicians be so dead-set
on it being just a conspiracy theory?
One cause is that it's an unproven theory, which actually does make many of the unproven claims about the lab leak "just a conspiracy theory". Such unproven accusations can greatly diminish the impact of the result of proper investigations, which could influence financial liabilities in the trillions.
Another issue is that the very government that would suffer massive losses if the world were to be convinced that the lab leak theory is true, consistently invests significant resources into preventing or disrupting all discussion and investigations that associate them with the origin of coronavirus.
China did so many things that made covid worse I don't think one more
really makes much difference
Being selfish, or even incompetent - which is what the linked examples accuse China of - is not the same as being directly responsible for causing a plague, lying about it, delaying the international response, withholding vital information about said plague, and then profiting from that information. These are multiple orders of magnitude apart in severity.
So why should any average people care whether covid escaped the lab in
Wuhan or not?
Because it's human to want simple explanations we can understand, to find a cause and someone to blame if something big happens. The prime example is humans in every corner of the world making up gods and pantheons (to the reader: most people agree that some religions are not "real". I'm talking about those religions, not yours)
The only explanation that makes sense to me is that Trump believed it
We don't know if Trump believed it - few if any people know what Trump believes. Trump did invoke this conspiracy theory, without evidence, which is highly problematic for a head of state. He did benefit from doing so because if the blame for the pandemic lies with "not Trump", it does not lie with Trump.