Why are the VHF and UHF airbands directly adjacent to frequency bands used by vital radionavigation aids?
The most obvious reason is that the ITU allocated the VHF and UHF bands for aviation purposes generally, and then aviation authorities had to split those limited bands into adjacent comm and nav sub-bands.

If aviation had gotten separate ranges for nav and comm, then that would double the risk of interference from adjacent non-aviation users, plus it would have made designing radios and sharing antennae more difficult.

So, we got the lesser of two evils.