Just because I can does not mean I do. How many passenger jets are build each year in Sweden (They obviously CAN build advanced jetfighters and do), how many nuclear subs in Germany (We CAN build nuclear ships - See Otto Hahn) Sometimes you don't need the stuff, sometimes it is cheaper to buy it from someone else.
The airports in Sweden maintain and repair those passenger jets, and that's essentially the same TL. The primary reason why they don't build widebody passenger jets is because the domestic market is too small and the international market is deemed saturated. If Sweden were 100M+ pop they'd build their own. Nuclear anything is a touchy subject in the Real World, so that isn't a very good example. If safe, cheap fusion power existed Germany would be building fusion power plants.
Just because some people travell to the stars does not mean all do. Same with other technologies. Germany 1960 was a high-tech nation again yet large parts of it still had no central heating, running hot water or communal sewers (i.e my father worked in the most advanced coal mine in the world yet had to stoke a coal fired oven for bathing water until the 1970s, we got a sewer access in the late 1980s). OTOH medical services where eccelent even then with access to the latest gear because it was needed
In this case part of the issue is innovation. Electric/gas water heat, central forced air heat, refrigeration, etc, were relatively new tech, less than 100 years old back in the 1960s. Not so in a broad interstellar milieu several thousand years old.
The other part is economic practicality, not technology. My Dad's house doesn't have sewer access even though it is in a pricey neighborhood. Every house has a septic system. It is far outside the city limits and will probably never have sewer access.
On the other hand, in a large city a septic system is impractical. You need land for the tank and drainfield. You can use cesspits that have to be emptied if a city doesn't provide a sewer.
Once you have certain infrastructure in place the tech involved is immaterial. We use electrically powered carnot cycle heat engines for cooling. It doesn't matter if the electricity comes from hampster treadmills, water wheels, coal-fired steam turbines, direct electric fuel cell, photocell, or Trav fusion. We could also swap out the carnot cycle unit for a Peltier effect unit, or some posited higher tech cooling method.
That city sewer system is connected to a waste treatment plant, with settling tanks, digesters, filters, and a landfill. It could be connected to a higher tech solution that transforms the waste into usable substances.
So, we dont have to sell gravcars to Ma & Pa Kettle, we get the government to replace their less reliable search and rescue helicopters with gravcars. Somebody then decides that a commercial service would be improved using grav vehicles.
If the demand is big enough, an enterprising local could obtain the 3I datapack and manufacture standard grav effectors for replacements. Then somebody starts making vehicle frames, and now you have a local air/raft industry. It doesn't have to become standard for average family.