But when joining a Zoom call from your phone you dial a number, then enter the meeting ID. The meeting ID has the same number of digits as a US phone number, but it isn't the number you dial. The calendar invites generated by Zoom format the number + meeting ID in such as way that a user can tap them and it will dial the number and enter the meeting ID.

Basically, in both cases (computer/app or dial-in), increasing the number of digits of the meeting ID has very little impact on the users. Forcing a user to enter a password after joining (which is just more digits) does impact the user.