Mainz was also part of Germania and is much further East.
The current extent of modern Köln is much wider than that of the Roman city of 20,000 people, so you'd have to dig in very specific places to find Roman artifacts.
In contrast to CCAA and Augusta Treverorum, Mogontiacum (Mainz) was not a colonia. It was primarily a military post. The main difference is that citizens of a colonia were in fact fully Roman with all the rights and duties. It was also part of a different province - Germania Superior, whereas CCAA was the centre of Germania Inferior.
Mogontiacum (Mainz) is further east than CCAA. One of it's purposes was to secure the Limes, but there was a comfortable buffer between the Limes and the town itself. Further north, the wilderness basically began when you crossed the Rhine at Colonia Ulpia Traiana (Xanten) or in fact CCAA...
You'll find the majority of Roman artifacts in an area roughly a third of the size of medieval Cologne, which itself is basically just the very city centre of Cologne today. However, a lot of digs were made along the roads leading out of Cologne since these were prominent burial sites for rich Romans. These roads follow the old layout until today...
Of course you're also right that modern-day Cologne is bigger than just the Roman center but that's a bit pedantic. The Roman center is of course full of actual ruins (sometimes with medieval ruins on top of them) but artifacts in general (e.g. coins) are still discovered anywhere in the vicinity and especially along the historic roads.
In the Roman city center it's enough of a problem that construction projects tend to be delayed extensively but even outside of it "Roman stuff" is still among the things you might accidentally find by digging, alongside more exciting things like live bombs from WW2 air raids.
EDIT: It's also worth mentioning that there was also a military outpost on the other side of the river in what is now the "Deutz" district.