Exactly. Airborne nukes are outdated. Landbased missile silos are still in use in the east-west Europe context as it's a contiguous land mass of more than ten thousand miles from the English Channel (western end of France) to the Bering Straits (eastern end of Russia). Why send the nuke by air mail it can be sent by e-mail? For the rest of the world, US, Russia, UK and France can nuke anywhere they like with ballistic missile launched from submarines, and nobody knows where these submarines are lurking with 70% of the earth being ocean.
During some stage of the war, the parties will start to search & destroy each other's nuclear arms. Therefore mobile launchers have better chance to survive and conceal in these phases, to remain alive and capable of counterstrike.
Mobile launchers includes land vehicles and submarines and these bombers. Bombers are the only ones that flies, that means speed.
Part of the game is tracking enemies' nuclear arsons. The idea is if you can manage to track them all, then your upper hand is to be able to destroy them all, to eliminate their counterstrike chance. Bombers are hardest to track & destroy in this game because of it's speed, vs land vehicles and subs. When there is alert of incoming attacks to destroy your nuclear arsons, bombers can evade the best, subs and land mobile launchers are slow to escape.
There is a standard procedure during impending nuclear strike alert, the strategic bombers are to take off to the air within a short time. Leave their bases basically to remain alive for counterstrike. Bases can be all gone once enemies warhead landed on them.