My understanding is that on the Bitcoin Lighting Network, both parties, the sender, and receiver must make a payment on top of the outgoing and incoming transactions. Why is that?
Bitcoin lightning network need both parties to make (receiver and sender) a payment on top of the transaction because of [... does not answer the actual question ...]
No, both parties do not have to make a payment. In fact, nobody
does a payment in the sense of spending BTC using a regular Bitcoin
P2PKH payment transaction.
In the vast majority of cases, channels are opened unilaterally, which means that just one party deposits funds ('make a payment' (?)) during the channel creation. The result is that the channel partner just has a balance of 0, so the channel is maximally unbalanced. That can be fixed as soon as you have more than 1 channel or by spending e.g. towards your own on-chain address through a
submarine swap or buying something online.