The ability to enter academia without a PhD varies substantially by faculty. In faculties that train people for "the professions" it is more common to encounter academics that have come from a professional background but do not have a PhD. For example, many academics in Law faculties are professional solicitors and barristers and their expertise comes from this background, rather than from a postgraduate research degree. Many academics in Medicine faculties are medical doctors who do not have PhDs (though they still have the title "Dr" from their medical degrees). The same is broadly true of other "professions" such as Actuarial Mathematics, some areas of Business and Commerce, so areas of Engineering, etc.
Having said this, there is no doubt that the situation is changing rapidly over time, due to a rapid increase in the supply of PhD graduates in all faculties (see e.g., Cyranoski et al 2011, McCook 2011, Larson, Ghaffarzadegan and Xue 2014). As the pool of PhD graduates increases, there is greater competition in credentials for academic positions, particularly at entry level. This seems to be leading to a situation where entry-level academics are expected to have a PhD. In some places this is now mandatory (see e.g., Gibney 2018, Baker 2018). Growth in PhD graduates and the resultant inflation of entry-level qualifications has been so rapid that there is growing concerns of an oversupply of PhDs that cannot be absorbed into academia
(see e.g., The Economist 2010). Senior academics who entered the university system prior to this boom have usually achieved enough in research and their profession that having this degree is not an important addition, so there are still many academics at higher levels without PhDs. However, for people seeking entry into academia at lower levels, the proportion of entrants with PhDs is increasing rapidly.
Speaking from personal experience (though not at US universities), when I went through university in the late 1990s and early 2000s (in Australia) there were many academics without PhDs, mostly in the faculties listed above. In the Law faculty at my university, most academics were professional lawyers, and less than a quarter held a PhD. In the Actuarial school, the academics were actuaries, and none of them held a PhD (though one was working towards it). Encountering an academic without a PhD was extremely common. Since this time it has become uncommon to encounter an entry-level academic without a PhD.