Question: I think more frequent salary raises would be good for employee engagement. Rather than an employee "holding their breath"/working for a year and asking for a $5000 raise, why can't the employee feel comfortable asking for a $416 raise every month?

I know there are strong cultural norms surrounding this, but i partly suspect that all of the norms originally evolved in reaction to administrative and technological realities of processing employee pay raises in resource-strapped HR departments. Are there known barriers: HR technology, Legal, Accounting, Executive mindsets (business training?) that interfere with more frequent, low quantity salary increases?

Answer: 
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> Are there known barriers: HR technology, Legal, Accounting, Executive
>  mindsets (business training?) that interfere with more frequent, low
>  quantity salary increases?
> 
> 
> 

Companies budget annually. They occasionally revise them quarterly, but seldom monthly. This makes is harder to give unplanned raises each month.

It takes a lot of work to go through the evaluation process that leads to raises. A lot of discussions, a lot of approvals, a lot of paperwork. No manager I've ever worked with (and certainly not me) wants to go through all of that more than once per hear. Most hate it annually.

Next time your annual raise comes up, you should propose that you don't get your full raise all at once, but instead get the same amount parceled out over a 12-month period. Be a pioneer and see how it works out. I can't see how this could be a good thing for you, but perhaps your company will.
Another point of human psychology to consider is that people prefer to get a raise that is actually meaningful in terms of a noticeable difference in their paycheck. So suppose the raise is 3600 a year which works out to 300 a month. Now you see a lower amount after taxes. So that works out to maybe 150 that you can actually spend. If you got the equivalent amount one month at a time, you would get a little more than an extra 10 a month every month, Works out the same but not so encouraging is it? And much easier to fritter away without noticing. If you are paid biweekly like most people it is roughly 7-8 a paycheck before taxes which equates to about 4-5 afterwards. This is not motivating to any person I know.