Can someone remind me why ML doesnt want to tell MC that he's the one ?

Not confirmed, just a guess:

Maybe ML does want MC to be with him for love and not bc of the "name". And I think MC also would rather be with someone that he loves, not because a "name" tells him to. ¯_(ツ)_/¯

it was shown that the ml doesn't care about love at all so i still cannot think of a positive and healthy reason for keeping up the lie. correct me if im wrong but i think the mc never opened up with anyone that if he were to get into a relationship, he wants it for the long run with intentions or marriage just like his parents'

I’m pretty sure he said it was because he wanted to see if a person can fall in love with their namer partner without knowing they’re named

He’s seeing this as an experiment, to see if someone unaware of being a namer will fall head over heels for his named like how the story sets up the, ‘when you meet your named you’re instantly attracted to them.’

I’m pretty sure in some chapter the MC said this was for entertainment and curiosity.

ML* whups I wish there was a edit button