Of course a TUM ID "smith" would be easier to remember. But LRZ has to provide more than 150.000 users with user names. We don't want to decide which user Smith would be "smith" or "smith42".
With the scheme used now (consonant, vocal, digit, digit, consonant, vocal, consonant, e.g. ka21bab) we hopefully have found an compromise which is easy to remember and does not disadvantage anybody.