Why doesn't the stealth cruiser have shields?

The point with the stealth cruiser is to basicly use well timed stealth activation as a shield.

Being is stealth almost guarantees that any shot fired at you will miss. Of course, no enemy will fire at you when you're already in stealth. (In fact, their weapons won't even charge during this time.)

The trick is to activate you stealth after they have already fired, but before the shots hit you. This can be easier if you pause the game after you hear the shots, so you know wether you want to attempt to stealth, or just risk taking these paticular shots and save the stealth for later.

Alternatively, use the stealth early to make sure you have your weapons charged first, so that you also shoot first. Then just disable their weapons. But you need to sure you can disable all of them. (Or only leave them something useless, like ion weapons.)

Of course, this alone will not be enough to keep you alive later in the game. You'll most likely need to buy at least some shileds sooner or later.

Overall, the stealth cruiser is a rather high risk ship. Played well and with a bit of luck, you can down most early opponents without taking a single hit. But once you start taking hits, you usualy take a lot of them.