Devs and PMs: why your passwords are restricted?

Hello everyone.

Time and time again, whenever I need to open an account on a website (SaaS or otherwise), I find passwords that are restricted in unsecure ways: "no more than 12 characters" (or something between 8 and 20) and only letters and numbers.

I use Bitwarden to store and generate my passwords, and I like all of my passwords to include letter (U/L), numbers, AND symbols, and I want my passwords to be 100 characters long for added security.

What is the reason for these resctrictions? By the way, not all systems have them, but I would say that at minimum 50% do, if not more. Am I missing something?

Thanks!