It's not limitless because there is a finite amount of hydrogen in its storage. But even more so because it uses a stupendous amount of hydrogen every single second.

So 1) Man made devices just need to be fueled with hydrogen, and hydrogen is very very common. Not limitless, sure, but very common still

2. We would require the tiniest fraction of what the sun uses to generate all the the energy we use, which means even with a smaller amount of hydrogen available they might last longer

Of course, all this is moot if humans don't manage to survive long enough to really do anything with fusion because of climate change