The river Eden is prone to flooding, creating a natural separation between Carlisle on the south bank and Stanwix on the north. In between, there's a lot of land that is conveniently located, but unsuitable for either intensive development or arable farming. There's the cricket club on this land, but also several parks, public gardens and other sports clubs.
When the baths were in use, there were also two separate settlements - the milecastle on the north bank forming part of the defensive line of Hadrian's Wall, and the civilian settlement of Luguvalium on the well-protected south bank. Being directly adjacent to the Eden bridge, the site would have been convenient for both settlements.
Carlisle has been a continuously occupied town since before the Romans, and continuously since. I don’t think it’s too surprising that we find Roman things underneath modern things (in fact I strongly suspect that we have yet to discover most Roman things because they lie under modern structures that are difficult/controversial to excavate).
Is anyone surprised how geographically stable these kinds of gathering places are?