Probably the argument is that they (1) host the software in the cloud which costs them money, and (2) they review software for e.g. security issues which costs them money.
> The Digital Markets Act (DMA) establishes a set of narrowly defined objective criteria for qualifying a large online platform as a so-called “gatekeeper”.
Anyway, the EU is coming for them:
https://ec.europa.eu/info/strategy/priorities-2019-2024/euro...
> The Digital Markets Act (DMA) establishes a set of narrowly defined objective criteria for qualifying a large online platform as a so-called “gatekeeper”.