why don't fighters "refuel" in between rounds?

Those gels are overrated. 'Re-fueling' is not as necessary as they want people to believe.
The gels are well-marketed, aimed towards the mass market of ordinary amateur into sport or fitness. For whom the product is mostly unnecessary.

But, no, if you are exercising or taking part in a sport for 30 minutes to 1 hour, there's absolutely no need to re-fuel.
The human body is quite capable of storing enough reserves.

Of course, they are beneficial to super-triathlons, and tennis matches that can go 4,5,6 hours, long distance cycling, endurance ultra-distance running etc. etc.

 

:rofl

 

Because a few body shots later and they might puke it all out.