First of all, how could you afford them? (They cost something like over half a million creds.) Secondly, why would you need them? There wouldn't be any challenge to the game if you could blast everything else in space to atoms with just one zap. Face a swarm of level 1 Red Hessions in a Sabre? It would be like taken a Kalshnikov to a water pistol fight.

There are mods that let you buy ships from other games though (e.g. Star Wars ships). The ships are still expensive.

But it does sort of bug me in the SP games when I can't buy a ship because I have to be level 20 and up. The SP story line ends at level 18 -- which means you can't buy them at all when you need them.

The money is no problem at all.

When you are level 10 you can buy a the best light fighter in the game (you have to go a bit out of your way to do this, though), equip it with powerful particle weapons and then loot all the wrecks or do $200k missions (reward plus loot) vaporizing Red Hessians for the Corsairs. I did just that for the light fighter path of my maximum-fun Campaign and I had about 1.5 million cash when I went to the place where mission 6 starts. This can also be done with the heavy fighter that can be bought around that time, but that ship is nowhere near as combat-worthy as the LF I mentioned and so it is quite a bit harder to stick one's neck so far out. But T-Hawk has done it anyway.

Also at level 10 you can buy the freighter with the largest hold size in the game and start earning money to the tune of several millions per hour. Some of the best trade routes are still closed but there are plenty good ones that are not.

One advantage of being extremely rich is that enemies start dropping valuable weapon loot and so you can get high-class weapons long before the game allows you to buy them.

As for the other thing: I had plenty of fun flying the campaign in a fully decked-out Eagle (the memory of my first run through the campaign was still fresh then ). I modded the game to make a fully equipped Eagle buyable at Leeds, loaded a savegame from the start of mission 7 and started blasting the hell out of everything that was hostile. It's a game, and having fun is the prime objective.