Because tech moves fast. For instance, about 10 years ago, cloud was big (and it still somewhat is). I assume a lot of students dug deep and got their masters and PhDs in it. Now, there’s a lot less jobs or a lot more competition compared to then.

Just change. It is amazing how fast an academic field can "burn out", for instance the discovery of the renormalization group in physics and its application ran a lot of its course in just a few years. Similarly if you were a deep learning expert 5 years ago departments were really hiring up and you'd have an easy time continuing in academia, if you start on deep learning today it is possible there are just too many deep learning experts and not enough jobs.

Because PhDs aren’t fast to acquire. It could take 5-10y to complete. And you usually can’t do a PhD part-time and continue to work.