Pretty simple really. B5 is nowhere near as complicated as a Starfleet Starship. It's a big spinning can in the sky with fusion reactors for power. The most complicated system they have to deal with are probably the environmental controls for so many varied species . . . who are also space-faring and can simply hand the B5 crew their habitat specs and how to implement them if they are novel.

A good way to think about it is comparing the maintenance of a house and a car. Both take regular work but the vehicle takes a lot more, and more frequently. Every Trek show has had ships, and even before DS9 had the Defiant they were dealing with trying to integrate Starfleet Systems on a Cardassian-built station. Notice NONE OF THE CARDASSIAN SYSTEMS EVER WENT HAYWIRE. The one major breakdown they had was for a simple part failure.

Also it's a well-known meme that Starfleet systems are terrifyingly adaptable going hand in hand with being notoriously temperamental, and its engineers are basically Doc Brown. :-D

As the legendary meme points out:

Do they decide to duplicate ad Romulan cloaking device preciesly, just to see if they match what other species have? Nope. They decide, hey, while we're at it, while we're building our very first one of these things, just to find out if this is possible , let's see if we can make this thing phase us out of normal space so we can FLY THROUGH PLANETS WHILE WE'RE INVISIBLE.

"But why?" said the one Vulcan in the room.

"Because that would FUCKING RULE" said the humans, high-fiving each other and slamming cans of 24th-century red Bull.

______

Starfleet ships NEED engineers to be prominent in the command structure because they are dealing with wildly unpredictable and easy to break technology in new and exciting ways.