Good point.I think a pretty big deal with the militia line is that switching into it is so expensive.
When pro's talk about the chance to switch into infantry it often gets mentioned that the switch would be too expensive and it really makes sense.
If you only count castle age techs, fully upgraded knights cost 250/150 (Chain Barding) and 220/120 (Iron Casting) + Husbandy (150 F) --> 620F /370G
Longswords Cost: 200/100 (Chain Mail), 220/120 (Iron Casting), Longsword Upgrade 200/65, Squires (100 F), Arson (150/50) --> 870F/335G
In conclusion you are paying more resources for upgrades and get a weaker unit in the end.
Its really not that surprising that infantry does not get played more often if you just look at the raw cost.
To be extremely precise I don't think teching into infantry is "that" expensive, it's more teching into Knights/Archer-line that's way too cheap