Why wouldn't it ?

It's not because it wouldn't work, it absolutely can. But it would take more time to make it work with a similar level performance, and that would mean taking time from making more online content. It would delay the online updates and pay features, thus holding back the other onlines.

To be fair, this is just a guess, because if they haven't ported GTA V by now after the Switch has proven its success, there HAS to be a good reason to.

Yeah but usually publishers aren't shy when it comes to delay stuff for the Switch (for eg. updates for Warframe)

Rockstar is probably not concerned with Nintendo

I am... GTA V sold more copies than Switch will probably sell on its lifetime and the sales just keep going. Most of their profit comes from online content now, not sales

Unlike other third parties they just see Switch sales and yawn at all the world it would need to be done to make it work on Switch, they are probably using their time to satisfy their already existing userbsse 

As someone who is not familiar with GTA V, does it allow cross-platform multiplay? 

Regarding your  GTA V question the short answer is no, the long answer is maybe, with the standalone GTA V Online that's due with the remasters being touted in some media circles to have a very high chance of adding crossplatform.

I see, thank you

With this shaping up really well, if Kingdom Come Deliverance and World War Z also turn out to be impressively done ports, I'd say Saber will have well and truly taken the throne for the best porting studio on Switch.