Well, in addition to the fact that the 5e PHB (for example)
continues to sell very well, there are a few factors-
1. People who bought early in the cycle ... well, they will want to
replace their old books. If you play a lot, the wear and tear eventually gets to them. Stuff spills. Pages and bindings get a little frazzled, etc.
2. Others will simply want a new version because their older books do not have all of the
collected errata and updates. It's a one-stop shop instead of having to remember all the changes spread across multiple sources.
3. Some people will be gaming with new entrants to the game, and those people will be playing with the new books. So you might want to buy them so that you can stay consistent.
4. Of course, if you are among the contingent of people who doesn't play 5e but enjoys complaining about it on this forum, you need to buy the books to make sure your complaints can be sourced! Or ... just keep referring to the SRD. Yes, other people notice.
5. Some people really enjoy new art, too!
6. Finally, this isn't a massive investment. When you compare the cost to, say, a major videogame, or going to the movies, or dining out, the cost of the core three RPG book as an investment in a decade of RPG playing seems ... more than reasonable.
Now, all of that said- everyone is different. But there will be a large number of people that will want to keep current- in addition to the constant influx of new entrants.