Because tha thu deiseil doesn’t mean you have finished.
deiseil is an adjective meaning finished, prepared, ready (maybe you are ready could be a bit clearer translation here?).
For you have finished you’d need a past tense verb, eg. chrìochnaich thu you finished; you have finished. You could also say tha thu air crìochnachadh you have finished (lit. you are after finishing, using a perfective construction with the preposition air, here meaning after, and the verbal noun).