I feel like you already answered your own question. These reasons may not apply to you, but they certainly apply to me.
No, I do not trust megacorps to keep my credentials safe.
I do not wish to use the same credentials for every site, for several reasons: in particular, worse consequences of account compromise; worse consequences of terms of service changes.
No, I definitely do not wish megacorp identity provider to be able to link information from my accounts across different websites.
No, I definitely do not want to be reliant on a megacorp. Amongst other reasons, I do not believe that a single entity can provide for all cases and competition in solutions is a good thing.
I don't want my account at random website X to be dependent on external services. Random website X will be slower and less reliable, and I will be less able to access support.
I do not want any single notion of my identity to have too much power if compromised.
I would rather accept the risk that random website Y is storing my password in plaintext rather than accept the risk of using a megacorp (or other external service).
No, I do not trust megacorps to keep my credentials safe.
I do not wish to use the same credentials for every site, for several reasons: in particular, worse consequences of account compromise; worse consequences of terms of service changes.
No, I definitely do not wish megacorp identity provider to be able to link information from my accounts across different websites.
No, I definitely do not want to be reliant on a megacorp. Amongst other reasons, I do not believe that a single entity can provide for all cases and competition in solutions is a good thing.
I don't want my account at random website X to be dependent on external services. Random website X will be slower and less reliable, and I will be less able to access support.
I do not want any single notion of my identity to have too much power if compromised.
I would rather accept the risk that random website Y is storing my password in plaintext rather than accept the risk of using a megacorp (or other external service).