Many accounts are tied to a phone number.

I still don't understand. Occasionally you need a phone number to sign up, but I've never needed a phone number to log in to something unless 2FA get triggered. But in any case, nobody else knows what phone number I used to create an account.

You often need two different phone numbers to create two different accounts on the same service. Otherwise you will get errors like "phone number already used, did you forget your password?".

WhatsApp

Switching sims doesnt log me out of whatsapp either - as far as i can tell its totally optional whether i want to update my whatsapp number to my phone number or not.

Yes it’s true that switching sims doesn’t log you out, but if you decided to intentionally log out from account A and create a new account B, you would need a second SIM card, that’s what the parent comment was trying to explain.

Makes sense.

I use VoIP numbers for that, although I understand it's not an option for all countries.

Yes, but you can explicitly log out from your main account and then log into an alt account - as long as you’re not using the sim associated to your main account