For one, a personal domain is less secure than a gmail.com domain. There have been quite a few targeted hacks, like Mat Honan or any number of crypto-holders, where a personal domain was socially engineered to be transferred to a new registrar (or password reset on existing registrar) where the attacker controls the MX records, thus your email, thus your password resets.

It’s much harder to transfer gmail.com, office.com, etc

That’s something I’ve been worried about since I switched to a custom domain email address that I point to another mail service: I protect my registrar account with 2fa, but I still think that the chances of someone hacking via social engineering my DNS registrar (Gandi) or the MX provider are much higher than someone hacking my 2fa personal @gmail.com account. I might just be paranoid.