That's something, I asked myself several times. But if you think about hero's, which help to built lightships or units in the barracks, than you pay for units and that changes the balance between the players very much to the advantage of those who can spend the money
That's true, but the cost to teleport can increase each time the hero moves.
For example, if you move Aristotle (light ship building hero) instantly to city A for 150 gold, then if you want to move him to city B it would cost more to move him instantly from city A to city B since you already moved him to city A instantly (could be double as an example).
Also, they could implement as a Beginners Quest like with the free Olympic Games, which would happen after the conquer a second city quest.
on speed worlds x4 /x6 its no problem to shifting assignation of Hero and managed build a army with event bonuses like Nereids .. but its a big problem on slow worlds <4 ..
for my opinion assignation should be stable and not depend of world speed ..
1h30min is really acceptable assign time hero to city . with good skills manage and a little lucky You can shift a Hero beetween 4-8 cities and queue build a army before end of buffs