I suspect they could, they just would rather not.

Ultimately I liked STR but it's only an hour from FRA to Stuttgart Hauptbahnhof by ICE (while STR takes the best part of 40 minutes). The logistics of having an operation there, even shared with other airlines, and paying lounge fees etc. out of those low fares, is quite difficult - while there aren't that many loyal passengers flying to Stuttgart compared to *A/LH loyal business travellers who are happy enough on EW.

It's a nice airport and convenient for some of my favourite parts of Germany, but it's easy to see that it's one of their more strategically vulnerable destinations, and going single-daily might work for price-sensitive leisure passengers but would undermine the business traffic that would be willing to pay fares higher than Eurowings (or at least a late-booking premium).

Same applies to CGN: the only strategic imperatives would be to do a spoiler operation like OAK was to Norwegian, or to create extra capacity on what is already a densely-served route network.