The same thing keeps happening to me when I play single player against the AI on normal difficulty.
I have no trouble taking the smaller planets, but as soon as I go to assault a larger type planet with several spire starships and usually at least a Fortress or two, all my fighters get demolished almost INSTANTLY. These are Tier 2, 3, and 4 fighters by the way.
My bombers survive practically forever, but then I have no means to destroy the remaining Starships, which just slowly but surely eat away my fleet. Occasionally, I can get my hands on some Anti-Starship Arachnids, but even 29 of these take a long time to kill a Spire Starship or Zenith Starship. I just have no idea how to assault a heavily defended planet without getting my fighters destroyed almost instantly and then getting molested by everything else.
Are fighters not worth using late game? If so, what do I need to bring to take out those pesky Starships?
edit: To clarify, it's typically the Fortresses and Plethoras of Missile Turrets that take out the fighters quickly, since those weapons have bonus damage against them, and fighters aren't hard to kill in the first place.
Thanks,
Wing