Do they? A quick mental scan of well know paedophiles suggests that famous ones face limited repercussions.
The sentence isn't the longest of any crime for the same reason it isn't death (or, for the jurisdictions without the death penalty, the same as their penalty for murder).
The difficulty of proving rape is why rape conviction rates are low. Imprisonment for rape is shorter than imprisonment for murder because most people consider murder worse than rape.
> The difficulty of proving rape is why rape conviction rates are low.
Even despite the low conviction rate, rape convictions are reversed at a higher rate than other crimes, because it's so difficult in those cases to know the truth.
> Imprisonment for rape is shorter than imprisonment for murder because most people consider murder worse than rape.
Many countries historically imposed the death penalty for rape and a few still do. Typically the ones less concerned about proof beyond a reasonable doubt in general.
People viscerally hate rapists and, even more, pedophiles. Murder is nominally worse but not by much. The penalty for rape and second degree murder are typically about the same. In multiple US states the penalty for rape is higher.
It's probably because sex crimes are extremely hard to prove. If there is a murder, somebody is dead and somebody killed them. It's not that ambiguous. Consent can be extremely ambiguous. It drives people mad because you want every rapist to be six feet underground and every falsely accused innocent person to be free but there are all too may cases, plausibly the majority of cases, where there is no way for the system to conclusively distinguish them.
And then we hesitate to kill rapists because we're uncomfortable about that when it's so easy to get it wrong, but if we don't, people feel the guilty ones are being insufficiently punished. This is kind of a recipe for ending up with bad laws.