Why are DIP through-hole package ICs not typically used at UHF and higher frequencies?
Why A dual inline package has relatively long leads running from the chip die out through the plastic body and down to the board. At UHF those leads act as series inductors and small antennas, adding parasitic reactance, unwanted coupling and resonances that ruin the circuit's intended behavior. Surface-mount packages are used instead because their tiny terminations keep lead inductance and stray coupling to a minimum.
Watch out Dielectric loss in the epoxy body is a real but minor effect compared with lead inductance, and the epoxy does not become conductive at any amateur frequency; that choice is simply invented.
At UHF, wire is inductor and antenna: short leads win. DIP legs are too long, so go surface mount.