And here's the screen. Presumably the Imperium preferred using jump-capable ships that might make it out over fighters that would be definite sacrifices. Note that the tender is worth 23 billion credits, the riders 9.3 billion each (so about 65 billion for all seven), and the fighters 105 million each (or 21 billion for the lot). A Sloan is worth ~3.3 billion, so all seven cost about the same as the fighters. So the escorts cost the same, and have fewer crew than the fighters, and you might not lose them all if they're used as a screen, while the fighters are gone (because you jumped out and they can't). Also, if the universe behaves in ways other than as High Guard says, the fighters aren't much of a screen because they can't actually hurt serious warships.
Also, when you're not fighting a fleet action, the Sloans give you options that fighters (and riders) don't, like being able to conduct extended patrols, be used as jump-capable couriers, and so on (though they'd be more generally useful if they carried some troops).
While ships for a game like Traveller should make sense in any rules for them (combat, trading, etc.), they should also make sense in the universe as it's presented, not merely within the limited focus of those rules. Modern warships have capabilities and facilities that make little sense in a straight-up ship vs ship conflict, as that's not all they do, and it follows that Traveller ships will too, especially when the combat rules are as artificial as High Guard.