The real recordings have some issues from creators' point of view. It limits our ability to iterate (newly added sentences would need to be rerecorded, and then added to the course by the Duolingo staff). It's also not a trivial task to record each sentence in the course and then individual words.
As far as I am aware, when real recordings are used, then it is recorded in a proffesional voice/dubbing studio, so tts is actually cheaper (both in Duolingo's money and contributors' time). Courses on Duolingo typically use TTSes when a relatively good one is available (like Spanish, French, English, etc.) Only otherwise the real recordings are ordered (Ukrainian, Hungarian, etc.)
As it is now, the Polish audio is crap. Don't shoot the messenger. Nearly half of sentences have a wrong intonation, with the voice weirdly soaring at the end. Sometimes it is the other way around when it actually should not, ie with questions. It is very irritating for someone like me, who know how to pronounce well. It is much worse for new learners, who rely only on Duolingo and basically learn wrong things because of this course.
These last days I've been learning Hungarian a bit and the difference in quality is huge. Granted, TTS is cheaper...