DIY also often has the benefit of being the only option when bureaucratic hurdles are in the way. Sometimes, approvals (from the government or from the landlord) are required, which can be hard, onerous, impossible or time-consuming to get.
Ignoring those requirements is often the most practical way with very limited negative consequences.
It also allows using much cheaper units - from what I've heard in Germany the unit (without the install cost!) will cost 2x as much when installed through an official installer compared to a high quality free-market option, but of course the installer will only install units sold through them so many people may be priced out of the legal route completely.