Why do people refuse to get Maw when they are against an AP heavy mages?

This item was literally built for that purpose alone. But more often than not, with the exception for zed talon and yasuo, not one gets any mr, they just go their respective "starting items" and get mad when they get bursted down. Like come on, build paths are not 100% set in stone. If you need MR to survive, they why dont you get some? Has anyone else experienced this? For me it usually is my ADC and my JG who refuse to buy it.