>>747224>>745356>halbs work really well vs paladins but arbs destroy elite skirms
Each one has a major shortcoming. Skirms can tank a ton of hits, and can force engagements against archer/spear compositions, but can't deal major damage back or be used for anything else.
Halbs can deal superb damage to cavalry, but they can't take a hit and can't force engagements.
Camels can force engagements and deal solid damage, but they cost gold, can't take a hit, and many civs don't get access to them.
Towers resist arrows entirely and deal amazing damage against archers, but they can't force engagements, require investment to be used, and are vulnerable to siege weapons.
This is by design, and it keeps counters from becoming absolutely dominant from the get-go.