According to David Crystal, the reason that words like "soothes" and "breathes" are spelt that way is to distinguish them from other words that are spelt the same way, especially nouns, in this case "sooth" and "breath". In the case of "smooth", there's no corresponding noun, and so the adjective is the same as the verb, like "wet".
This is more an observation than a rule. If I hammer a nail, I do it with the noun of the same name. If I heat a cake, I do it with the same verb as the abstract noun that the cake acquires. And so on. It probably to do with the time when these words were first written down in English, and the spelling fashion of the time.
How freaky, I was pondering this EXACT thing only a few minutes ago... I wrote 'to smooth' in the context of to smooth dd's hair and then deliberated about adding an 'e'. I refrained, but am not sure...
(I always found it surprisingly easy to see what I was cutting using a pair of baby nail scissors - less chance of removing end of finger in case of sudden movement too . I wonder if they might catch on...)