Possibly because a trim gauge may not be that useful.
I experimented with an analog trim gauge on my CC (yes, it's a Merc OB, NOT an outdrive like you have, but perhaps you would get a similar result). I found that the entire useable, "while underway" trim range was gauged within the bottom 1/16 or so of the needle sweep...the remaining 15/16 of the needle sweep was already into tilt territory. (And yes, my sender and wiring is all fine and reads correct sweep of ohms, that's just the way the gage reads)
It would be like if you had a 32gal tank, but your fuel gage measured only the top 2gal in the tank, and sat on "E" for the remaining 30gal. Not very useful.
(Now, if you're talking about digital / SmartCraft, maybe that could be "calibrated" and/or "truncated" into a useful trim readout. Or maybe a "high end" analog trim gauge would allow you to calibrate the sweep. Or maybe if you're an EE you could design a "resistance multiplier" circuit for your analog sender. But these are all things outside the scope of what I know about.)
As achris and GaryDoug are alluding to, you'll use trim (for one example) to prevent "porpoising" (i.e., trimmed out too far) or broaching / "nose-diving" (i.e., trimmed in too far) depending on the height and period of the chop you're cruising through, and that's a mostly "by feel" thing anyway--as in, you tippy-tap the trim switch a tiny little bit at a time, and when you go from either bow "plowing under" (or bow "bouncing around") to "rock steady," you're trimmed right. No gauge needed.
