Small creeks do hold fish - salmon, steelhead, SRC, etc.

Why don't they get much pressure? Here's a few reasons:
1) Timing is even more critical
2) Fish are easier to spook
3) Access generally sucks - unless you like hacking through salmon berry, black berry, and devils club... Just to get to 1 hole, then do it 5 minutes later to try to find another hole
4) If you just spent an hour hacking your way to your 'secret spot', and find someone already there, that would really suck!
5) As noted earlier, the waters are often closed, because these fish are probaly actively spawing and easy to harass
5) And most of all, the experience, while fun, is not what steelheading is about for most of us - we search for that 'classic' steelhead water, with the long deep runs, tailouts, slots, drifts, deep pools, riffles, ect. If you don't catch fish, at least you enjoyed your surroundings...

But if I lived by a smaller stream that was open to fishing, I would certainly find a few 'hot spots' to hit when the larger waters wern't working for me.

Have fun exploring...