mempool – What makes it difficult to prevent transactions where value transferred is less than the fees paid?

At the wallet level

Nothing prevents wallets from implementing reasonable safety measures to protect their users from undesirable behavior.

Bitcoin Core for example has a feature to (by default) reject the creation of transactions with a very high fee value. There is no specific rule to prevent fees higher than the amount transferred, but there certainly could be. Other wallet software may have similar and/or different preventative measures.

At the policy level

Policy rules exist to protect the network, not the individual node operator. A rule to prevent too high fees doesn’t belong here, as it doesn’t hurt the network.

Furthermore, it is hard to imagine what such a rule would look like, because the amount paid/transferred is by design not visible in the transaction (most transactions have change that is sent back to the sender, but which is indistinguishable from the “payment” output to outsiders).

The dust relay rule is intended to disincentivize the creation of transaction outputs which would be more expensive to spend than their value. Such outputs are likely to remain unspent in the UTXO set forever, slowly accumulating and eventually increasing node operation costs.

At the consensus rule level

The same considerations apply here as for policy rules, except they need to be agreed upon by the entire ecosystem, and are such much harder to change.

So far, this has meant no rules at all that relate to fees (apart from requiring them to be non-negative). This is in part due to the fact that what is “reasonable” economical behavior may change over time, at timescales much shorter than those over which consensus rules can be adapted.