Most things in aircraft automation systems depend on sensors that could fail (those sensors could be redundant etc. but the same can be done with the sensors on landing gears). It is sane design.

> What if the landing gear don't drop?

I don't think that there is need for reverse thrust in such a scenario.

> Is there a situation where you would still want reverse thrust without landing gear?

Even if there is, there could be a manual override.