Why the owner of a database can't alter the tables of this database that are owned by other owners?
It is a security consideration. One way to get around it is to have all objects created by specific user(s) have default rights.

In my opinion you should limit what user(s) can do DDL in a database for a number of reasons. Primarily for security and controlling who/what user can do DDLs. For a small company / personal setup may not matter, but the second you have multiple people working on a DB can get pretty chaotic if you don't control it.