>why, for example pg is the only one in my knowledge having this vacuum issue
Because other DBMSs have different MVCC implementations. Basically they keep that "snapshot" row versions in other places, not in the table itself. For example, in Oracle they go to undo segment; in MSSQL to tempdb (or you can just disable MVCC); and in MySQL with InniDB to rollback segments similar to Oracle (not surprising :).
Other MVCC implementations have their own drawbacks of course.
Because other DBMSs have different MVCC implementations. Basically they keep that "snapshot" row versions in other places, not in the table itself. For example, in Oracle they go to undo segment; in MSSQL to tempdb (or you can just disable MVCC); and in MySQL with InniDB to rollback segments similar to Oracle (not surprising :).
Other MVCC implementations have their own drawbacks of course.