Many do. I have a Fairphone which does, and my previous Huawei did too. It's a feature I tend to look out for when I replace my phone, precicely to avoid the issue you're having!
Most of the Galaxy S series do take SD cards, unfortunately the S21 is one of the two that don't.
If you don't want to get a new device, you might want to look at what's using most of the storage. If it's photos and videos, you can probably dump most of them to your google drive to free up space.