It's not really security through obscurity. In these case I understand the ids where related to data that the company was making available to users through emai

It's not really security through obscurity. In these case I understand the ids where related to data that the company was making available to users through email links. A cryptographically secure 128bit UUID is impossible to guess, no more than a cryptographic access token. Now of course, you would probably rather want to have an authentication scheme on top of that, but that comes at a support cost in term of customers loosing their passwords, locking themselves out of their account, etc. And it is not clear you have increased security as people re-use passwords. Then of course there is the issue that email is for the most part un-encrypted (or encrypted without validating certificates).