Why PS4 and PS5 players cannot play GTA Online together

The simple reason why GTA Online fans cannot play the game between PS4 and their PS5 is that there is no enabled crossplay between previous and next-gen consoles.

GTA 5 Online Interaction Menu: How To Open & All Options List

Gta Online Character Transfer To PS5 Or Xbox Series X

I'm Playing Grand Theft Auto V For The First Time And It Lives Up

Grand Theft Auto 5's 'next-gen' upgrade is the best version yet

PS5 vs PS4 Pro: should you upgrade?

Grand Theft Auto V (US)

Don't expect to transfer your GTA 5 progress to PS5 or Xbox Series

Grand Theft Auto V (PS4™ & PS5™)

Grand Theft Auto V - Wikipedia

Is GTA 5 Crossplay? Cross-Platform Support for PC, Xbox & PS5