If you read the relevant docs you will see how this is implemented and maybe realize that locking your entire database for a write transaction isn’t something that works for a whole lot of cases. Of course multiple readers are allowed, but multiple writers even to different tables still contend for the same lock (that doesn’t work on all file systems). There isn’t table-level locking, let alone row level locking.
Excuse me? The only way you would be able to come to this conclusion is if you did no reading whatsoever and never tried it.
This “confidently wrong” attitude really needs to stop.