It's probably to keep it easy for end users to create content. SL had the same issue. Pro game creators have way more money and skills than ordinary end users,

It's probably to keep it easy for end users to create content. SL had the same issue. Pro game creators have way more money and skills than ordinary end users, so unless you want a really jarring set of contrasts between ultra-basic objects and super awesome objects you have to pin everything to the minimum visual quality. Sort of like why Twitter is popular - nobody can excel at anything because of the character count limit.