This is something that is debated quite often.

One of the other arguments here is that when it comes to games I can get a Nintendo, or an Xbox, or a PS5, or a PC that I can run Windows/Linux games on.

When it comes to phones we pretty much have a duopoly which means both groups can find it easy to manipulate the market to higher prices without indirect (illegal) signalling.

Luckily it looks like the EU is telling Apple to suck it and they'll have to open up app stores on their phones.

All of those gaming platforms have platform exclusives though, and pretty clearly can exert significant influence on game developers due to their ability to determine what software runs on their devices. I'm not really sure how the duopoly of smartphones is significantly different than the handful of competitors in the gaming market.

There are thousands of competitors in the gaming market.

There is also a big difference between a company saying "I am only going to make this for the Apple app store" and saying "You can only have the Apple app store".

No, that is also a problem, and yes exclusives should be illegal. Why?

How would you go about making it illegal for developers not to do the extra work to port to every platform?

I would only make it illegal to sign an exclusive contract. The market would handle the rest, unless the platform they targeted had unique capabilities that other platforms didn't (e.g you game will only work with on a touch screen).