Why do some Christians believe we must be formal and "reverent" in the church building? What constitutes reverence or respect and what constitutes casual?
Excellent question! There's a difference between "formal" and "reverent". Most Christians would agree that there should be at least a cursory level of revence in church (or, at least, in the worship center of the church building). However, what constitutes reverence varies wildly from region to region. I mentioned about having been to churches in Hawaii. Many churches are, quite literally, beachfront. So when worship starts, people will literally will come out of the water into church. Shorts, bathing attire, and shirtless men are a common site in such instances, and there is no irreverence whatsoever. OTOH, if I attended worship at some presbyterian churches, anything less than a nice pair of slacks and button up shirt might be considered irreverent.
If you're a Christian in some parts of Papua New Guinea, it would be the height of irreverence to wear a belt. Now, keep in mind, that in those same areas, a belt is pretty much all people wear (both men and women are topless). On a mission trip to Indonesia, I was required to remove my shows and leave them at the front of the church, because shoes in the church was considered irreverent. OTOH, NOT wearing shoes is considered irreverent in most US churches.
Now, in regards to formal vs informal, it's likewise regional. "formal" should never be considered a synonym for "reverent". However, in some parts of the country and world, formal attire is considered appropriate attire for being reverent.
This truly is acase of "when in Rome".