Because the reason monk/mangonel works so well is because of the range of monks.If we are talking about playing vs meso civs, then monks mangs are a terrible idea and I'm not disputing that. They are slow and fragile and ew completely destroy them. (some exceptions in aztec war when monks have ew support).
Vs non meso civs, the difference is that longswords are terrible vs kts, whereas monks are very strong vs kts. In addition monks with redemption allow automatic wins in a mang war; assuming decent micro.
Tbh I'm not quite sure what point you are trying to make.
My point was that, if Monks + Mangonels is good against non-meso civs, then why isn't Long Swordsmen + Mangonels good vs meso civs?