Masturbating = Insular activity, accomplishes nothing but dopamine rush, low risk low reward (i.e cowardly) activity. Infinite access to it means that it will never serve as a motivational goal.
Fucking = Necessitates a connection with other humans (Not necessarily an emotional one), an activity that you can possibly fail at (i.e, one worth doing.) Dopamine rush occurs as a result of using a comprehensive set of skills and abilities rather than just happening because you decided that it should happen. Limited access to attractive women forces you to improve yourself physically, mentally and/or financially in order to reach the goal.
You can overdo intimacy with other people as well, but the hurdles that have to be climbed to fuck someone else are a well balanced barrier to the dopamine hit that you are seeking. On the most basic level - you can masturbate while living in squalor in your mom's attic - but what if the woman of your dreams, for some inexplicable reason just decides that she's interested in you sexually? She still gonna be interested if you have to bring her to your mom's house and fuck her next to your frozen TV dinner trays? Of course not. You wanna fuck her, you gotta have a job and have a place of your own. If not, no matter how lucky you are or how many Chads vanish from the planet, you're still just gonna be jerking alone.
Look - you shouldn't eat a cake every chance that you get. But if you're willing to make the fucking cake every time you eat one, it balances shit out a little bit. You may not benefit that much from eating the cake (But maybe you will) but at least you will have put in some practice and improved your baking skills, so you maybe that's worth a reward. That's what cumbrains don't understand. They found a thing that is nice on occasion and under the right circumstances and abused their access to it.