To a large part it is probably simply uninformed developers choosing permissive licenses, because some of their favorite projects do so as well and that is what they know as "open source". The thought seems to be along the lines of:
"I am going to make it open source! What license was that again? Ah MIT license! OK, done!"
Only later, when a project has gained traction they might or might not realize that MIT license allows people to do things they would not like. But by then they need to ask all the contributors for a license change and it typically doesn't happen.
Often the people also think they must not upset companies, if companies uses their software. Sometimes there is also financial motivation behind that. Big players invest into that project directly or conferences or other things, giving the people involved in the project some fame. See for example project Jupyter. One look at the $ponsors and you know why they will never change to a copyleft license.
That is alright, but people should not be surprised, when the rug is pulled away under their feet and big tech creates some closed source alternative or derived work under a different license, that integrates with their other stuff and that the original authors do not see a penny of, even if millions of people use it.
When I decide on software to use for myself, and I have a good choice between something copyleft and something MIT or similarly licensed, I usually go for the copyleft one, because I have no interest in the corporate involvement.