As we know that almost all popular games are available on all gaming devices. But Grand Theft Auto V Online isn't cross-platform for these 5 reasons...
There are a lot of code differences in the game and hence Rockstar has to rewrite the game code. It is a time consuming process and of course it requires money.
Sony and Microsoft are the two main console manufacturers. But the both have completely different agreement, resulting in increased development costs and game time.
As everyone knows GTA 5 has a lot of game mods which change the whole game. If the game will be cross platform then all mods will also need to be modified to work with every device.
One of the major problems is the hardware of the gaming devices as they are completely different from each other. Some devices support up to 15 players in a single lobby while some devices can only support up to 30 players due to hardware.
May be they are planning cross platform in GTA 6 and hence they don't want to waste their time and money on GTA 5 because they are working on GTA 6.
So, the conclusion is, to make GTA 5 online cross platform the developer will have to work on it for years and that could result in better hardware as well as an increase in the size of the game in the end.