To be fair, I have seen it a few times.
Aside from a handful of meme Dinkle rounds that all happened within about a week, it virtually always happens on Trijent, with one or two Ice rounds thrown in. Notably, both of these are maps where marines pushing before the Xenos have pretty much already lost is usually suicide so long as Xenos follow the "meta" for those maps - hide in the underground/excavation caves in Ice, and let the marines push halfway across the map before encircling them in Trijent. On maps where marines attacking isn't a fools errand, marines virtually always push out as soon as the map lets them (*cough* LV fog *cough*), and only withdraw to their cadelines when things start to go south.
Now, what DOES happen all the time is that Xenos gain the upper hand the initial engagement once they commit seriously (which they kinda have to because the marines have zero reason otherwise to not keep pushing all the way to their hive), then marines make a semi-orderly withdrawl all the way back to the FOB (which they kinda have to because if marines wait for things to get bad before retreating then it usually turns into a route that gets over half the retreating force wiped). From there, the fight turns into trench warfare and pushes/counterpushes around the front cadelines, because marines are too slow to organize, too slow to move and too situationally unaware to reliably maneuver as a fighting force in ways that Xenos can't effortlessly avoid or counter, and because Xenos almost never let up on FOB assaults for the 20 minutes it takes for marines to organize an assault until Xenos have lost the majority of their force or until they force an evac.
There is also the issue of late game in rounds where marines have mostly won with heavy losses, but they only have ~60 troops compared to under 10 xenos. Unfortunately, these xenos will sometimes include several t2s/t3s and the queen staying together, which by themselves require 20-30 marines to defeat reliably, so marines have to choose between spreading out and getting picked off in small groups by a xenos deathball (and yes, I have seen benos bounce back from ~4 xenos and win by doing this), or playing extremely cautiously which drags the round out.