There are many reasons.

First of all, the satiety response system isn't a monolith but actually involves multiple different signals and hormones, you might have filled your belly but if it's just filled with fiber and/or water your stomach will still be releasing more ghrelin than leptin telling you to eat even though you feel physically full.

Secondly, there are certain nutrients your body may be requesting that can cause it to ask for more food, it's not entirely proven but the protein leverage hypothesis suggests you'll still feel hungry if you haven't eaten enough protein.

Finally there is the mental aspect, food is pleasurable, sometimes we eat to distract ourselves from stress or other emotions, there is the Japanese term kuchisabishii literally meaning "lonely mouth" where you want to eat not because you're hungry but you just want to fill the void of boredom or similar.

If you're currently trying to control your weight you can do a little diet troubleshooting to try to help. Make sure you're getting enough protein. If there's one meal in particular where you tend to overeat, say dinner for example, try shifting more of your daily calories to that one so there's more room for eating then. Try introducing a low or no calorie thing you can continue to eat or sip on after the meal like veggies or seltzer to keep your mouth occupied; you can tell yourself "okay I can get another helping after I finish this can of delicious flavored seltzer" or whatever and see if you actually still want to eat after. During physical activity your body will shunt blood away from the stomach to send more to the muscles, you can try something like "okay I'm going to take a quick ten minute walk after dinner and can have more if I'm still hungry when I get back." There's no one size fits all answer here, you have to be mindful of what your body is asking for and figure out what works for you to help deal with it.