the reason why we don't see anything under 1sat/vB is because default minimum value is that.
A slight correction - we do see transactions under 1 sat/vbyte quite regularly, even now when the fees are what they are. Miners are free to include any valid transactions in their blocks, and paying zero fee does not invalidate a transaction - it simply means it will not be relayed by the vast majority of nodes. Miners can include zero fee transactions if they are incentivized to do so via other means, such as someone paying privately or when the transaction belongs to the miner themselves.
For example, there are a whole bunch of transactions from this address paying zero fee and creating a bunch of OP_RETURN outputs, which I assume is some sort of ordinal spam:
https://mempool.space/address/1JRhv7zRN9xCyTntYT5nuupg7JMsE7YocL.
Or this transaction, which is F2Pool paying out mining rewards to individual miners:
https://mempool.space/tx/12d7d9678f98f319fa4c6b4295c5364a1b9c937901e6d83bded4533e523a319a
