Because, in a nutshell, it makes higher-level spells more effective, and caster's already have DCs increase by ability score increases et al.glass said:I seem to recall a while back, someone posting a very good explanation of why save DCs are based on spell level and ignore caster level. I thought at the time, 'ah yes, that makes sense'. Unfortunately, I can't remember what it was.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.