Because they would abuse it. We have hundreds of thousands of little kids who want to post “give me free in-game credits” or “hi [well-known user]! im you’re biggest fan!”, regardless of context and whether it’s appropriate. Our compromise was that these users aren’t patient enough to stick around to meet activity requirements, so those that were mature enough to stick around got bumped up to TL1. From there, users in TL1 could learn the ropes, as they couldn’t previously when all they had access to was likes/votes.
As Robert mentioned though, we can use a group for distinguishing between impatient/learning the ropes though, so we don’t need to use another trust level in this case (unless we want to do something like give TL0 learning the ropes access to PMs or similar).