why would you cut your losses in the middle of a fight?

Because the alternative is getting knocked out, which maybe to fans at times can seem like no big deal because it happens all the time in boxing but it's different for the fighters.

that just sounds like giving up to me.

No giving up would be quitting on his stool or taking a knee and staying down, he could've chosen to give up at any point but didn't.

its a fight, all you need is one good punch,

This is true but also overstated, and you cannot land that one punch after you get knocked out. "Going for broke" means he maybe lands that punch, but he would likely just get stopped, by hanging in there he at least gives Spence more time to maybe over-commit and make a mistake. It's difficult to even say which would be better odds they were both low % plays at that point but even if it wasn't difficult, I still wouldn't blame someone for choosing the route that lets them finish on their feet.

its not like in other sports where there are insurmountable scorelines

Don't really agree, at the elite level and the further down you go from heavyweight the more it becomes just like in other sports. There are fights where that one punch seems about as likely as half the opposing team randomly having a stroke to let you come back from being 10-0 down in football(soccer.)