In most cases because process improvement suggestions by non-managers (or in the worst cases, by anyone non-external) tend to get ignored by the upper chain or, worse, being stolen by someone along the chain.
Or, to put it shortly, why invest mental energy into improving processes when it's either being discarded anyway or used for the promotion of some manager without delivering any benefit to oneself?
And to make it worse, some organizations are actively set up that way to discourage internal improvement ideas so that the uppermost management is free to hire the infamous Big Four "consulting" agencies to mask what upper management wants as "external and independent" expertise that must now be followed. The Big Four (and their smaller fellows) are a fucking scam set up to enrich the top level at the expense of everyone below.
If you're trying to improve a process just for the recognition, then you're by definition not subject to it, so you're playing the role of a manager. And you excluded that yourself in your first sentence.
Simple: when hired as a developer, I expect being compensated in one way or the other for work that goes beyond scope - especially when it is like in most private companies where you need some form of achievements to show to get a raise or a promotion. And do not forget: when you improve processes, usually your workload will be increased as a result, so in the worst case you'll end up with your manager getting the raise for your suggestion and double the workload as "thanks".
You can fire or train those developers. You can't do that to the CEO.
Agile tranformation requires buy in right to the top. I've seen it work from the top down, never from bottom up and never from middle up/down either.