Waiting 20 seconds can be the difference between a quick attack they dont expect killing 4 guys, and facing a town with pikes at every entrance rendering you useless.
My 20 second comment was a bit of exageration. But it does take some time for 50 guys on foot to clear the spawn, especially those maps with one narrow path like the docks. Shoving through them is rude. Everyone has a pike as soon as they spawn so I'm not sure what you are talking about. Its not like they set up a perimeter pike wall or something to prepare for cav. All it takes is one guy paying attention, with a lance out to dehorse someone. More like all the cav hurry to see who can get the afk kills first.
Either way, its not a huge deal for me. I have more issues with cav parking their horse at an entrance, ladder, or staircase. And cav ramming into an infantry fight.