Because getting yet another hero faster does not make you more powerful in any way. There are over 100 heroes and dozens of good ones there is plenty to chose from for lesser spenders and f2p. Precious runestones on the other hand do make you considerably more powerful.
You're not seeing it from the perspective of a server that just made it to lvl 90, while I'm going to take 2-3 weeks in one awakening, someone that buys the awakening item/potion will have 2 awakened heroes. For example, this means that maybe I'll have Octo as my first aw, this other guy will have Octo and Garrick; by next DP i'll have Octo and Garrick, the other will have Octo, Garrick, Goram, Vernos... making him even more overpowered at least for long time. Of course after a long period of time it balances out, just like the biggest whale on my server who was unbeatable, 7 levels ahead of me (now only 1), 7 star VIP heroes, as we approached level 90 he slowly turned into a strong opponent but quite beatable by a few in the server (he was a big spender but a bad player, eventually he rage quit).
It's a complicated balance, I mean, what makes a whale happy? staying undefeated at the top without any game effort eventually makes some of them bored and they lose interest in the game, and if non-whales catch up to them some get angry and rage quit or some start posting rants that this is turning into a game with welfare XD (actual rant from someone on fb).