nothing prevents people from upvoting comments, it just seems most people are not doing much of that and using all their voting power on the main post

Yes in many posts it is in the comments where the most contribution/value/content is. 
In some cases people do not need "incentive" to make a quality in depth comment, but in many cases over time, if there is no "reward" for in depth comments they WILL happen less. 
Some of the problem lies in how steemit works but most of that actually lies in the people that either do or do not UPVote comments.
If more people chose to UPvote comments than they would be worth more. 
For example one of your comments on introduceyourself  @charlieshrem made over $401!
This does happen but is rare. 
SO maybe there needs to be built in code that provides incentive for more UPvoting of comments or maybe the Steemit community will just choose to do it more. Either way the potential does exist to give more value to comments thus inspiring more value in the comments them self!