I’ve been a DM for many years, and in all that time I’ve had maybe three players read the PHB. I don’t mind explaining the rules, but it would be nice to not have to remind the wizard how spell slots work (again). Is this a common thing for most groups?
In my experience my players are excited about the rules and how their characters work - but not all of them learn well from “just reading the rules”, like it literally does not stick. Just telling them to read the rules isn’t going to do anything but make them feel shitty about the game.
Talk to your players about how to help them remember or have easy access to these rules. Make sure you don’t get too accusatory, you do want to play with these people after all.