I have a couple of theories on this. First, many of the people who are choosing to "act" are doing it for the wrong reasons. They want to "famous" and make Lots and Lots of money. Back in the day, you had to have some sort of talent to make it and get work. Dancing and singing along with being able to impersonate a character some made for you to play. Now a days, all you need is a pretty face and some money or a connection.
Second, they really don't have to act at all, even if they had talent. Computers and make-up artists do all the work.
Please remember, this is not all of the stars today but a large majority of them.
There have always been bad movies, but they are now forgotten. Think of all the 50's, 60's, and 70's B-movie schlock. Although, some of them can be good. Then there are the direct-to-video 80's and 90's movies. Plus, nostalgia plays a big part. However, good movies now are few and far between. I blame Millenials.