When you sign a legal contract, you are also likely to spend a non-negligible fraction of the contract to lawyers so that they review it thoroughly. Think of the legal costs of fund raising or the time spent negotiating and reviewing a contract for a large deal.

Some things that are at stake on the lawyers side are their competence (can they actually make sure the contract is "secure" ?), their reputation (track record of competence established over time) and some insurance mechanism (if things go wrong can you get something back from their insurance).

All of the above seem to be missing in the case of "smart-contracts".