The spec dictates that the htlc amount for the final hop must match the amount in the onion payload exactly:

Is there a reason why the final hop must not accept an htlc with an amount that is higher than the payload amount?

Relaxing this requirement could be helpful to distinguish in a route description between the intended payment amount and the actual amount delivered to the final hop.

For example when the final channel has a min_htlc policy of 100 sat that is above the payment amount of let's say 80 sat, the sender could decide to use the channel anyway. That they are giving away 20 sat extra isn't visible in the route description. For the two last nodes, both will have an amt_to_forward of 100 sat. WIth MPP, there is the total amount, but it doesn't tell you how much each htlc is overpaying.