Because each mob has a different level (and therefore health), stacking mobs would be much more complex than it is on other servers. Also, each spawner is limited to having 3 mobs alive at any given time, and they generally spawn about 1.5 mobs on average. The only time when there are a high number of concurrent mobs is when you have lots of spawners, and when you have lots of spawners, you're almost certainly using an AFK grinder, meaning the lag is being cleared (just for reference, we have about 100 spawners on our co-op island, and the lag is no different to my solo island which has just 10 spawners - this is all on Bedrock).