there are 3 characters who can interact with the menu, Sans, Asgore, and Flowey/Asriel. People said that Flowey can only manipulate said Menu because he gets souls and becomes more powerful, but I don't think that's it. For instance, Sans can attack your menu options despite being the "weakest monster".
Also keep in mind Flowey had the option to save and load before you did, meaning he had access to more "menus" than other characters. This implies he is just as capable of messing with menu options as any other character.
Now, think about this, your character uses menus to equip items, yes? Yet these menus and menu options you have are all things 3 other characters have shown to be able to manipulate in some way, right? So here's a crazy idea for you, and my theory:
All characters in Undertale use menus to do everything, from eating, sleeping, and wearing everything. When Sans does his ultimate attack, which is "nothing" I want you to realize something, that during that fight, he was attacking you while you were in your own menu, and that the one attack that works on him, its after you used your turn.
That's right, you use Sans own method of attacking during the opponents turn against him. Your character can move without your input at times and here we see him fighting the weakest monster who does something nobody else does. Your character learned how to beat Sans, by FIGHTING Sans. Just like you, the player, did.
Crazy, right? Anyway, just imagine the fact Asgore could destroy a menu option while Sans could only attack them, and that its been noted by Undyne that he can dodge, with one final note saying that no human has EVER made it past Asgore, despite humans supposedly being so much stronger than monsters.
Tl : Dr
Monsters have the same menus than the player does, and they're a part of the world rather than just buttons you push.
Which, leads me to my next theory, which is why, exactly, humans are stronger than monsters.
The reason why humans are stronger than monsters, is because of MENU MANIPULATION. That's right, its not because their souls persist, or that they hit harder, keep in mind that killing intent and will to fight both affect your opponents, and YOU the PLAYER. This is shown by the fact that when you trust Flowey, he reduces your HP to 1, but in human-like fashion, your soul persists.
As a human, you are not above the rules that we kill monsters by. What is it that Frisk does that makes him so much stronger than every other monster, and is something no other monster does?
Item consumption, and the ability to dodge.
Keep in mind, no other monsters eat, but some can dodge, specifically Sans and Asgore. Both of these two can attack menus, Flowey/Asriel doesn't dodge, but I think that's because in each time you attack him, there's some heavy menu manipulation occurring. For instance, your option to fight him in his omega form is summoned up, and I doubt he's the one doing that.
The other time he's literally God and laughing at you for trying to hurt him, and the third, you didn't even initiate a fight with him.
Anyway, I think dodging and eating are menu options humans naturally have. When you attack a monster, they likely don't have the option to move away from your strike because of the rules of a fight. I think one of the first mechanics for a monster to be able to fight a human they have to learn, is attacking outside their turn, which is why Asgore was so excited Undyne managed to knock him down the first time.
Because he was dodging like a human.
Undyne likely COULD dodge as well, but her mentality is to take everything head on and face it, so she probably never cared to learn how. Anywho, humans can naturally dodge, and have the option to eat during a combat, which means they can mitigate damage far better than a monster, and therefore dish it out as well, plus the fact they can persist with their physical bodies, means that they can recover from a fatal blow.
Even children have this ability, and its very clear even children like Frisk can naturally manipulate menu options. I'd imagine a fight between two humans in Undertale would consist of a lot of mechanics juggling and button attacking. Would probably be crazy as hell.
Tl : Dr
Humans have more menu options than monsters naturally, asgore lives even now because he got them. Sans can fight because he learned them.