why should the game balance be calculated based on the player's desires?

Because there's Heroes 6, where the balance ignored the player's desires in favor of 2-level perks, which were "balanced."

Besides, in addition to the group of players who play without losses, there is a larger group of players who play with losses (because they play worse), as well as a smaller group of "pros" who play with losses (because they compete for speed and take risks with strong guards). For both the second and third groups, defense is much more enjoyable as a base.

And if the developers thought...

The developers didn't think very well. The balance, in terms of numbers, is not well done. This is clearly seen in the alt-upgrades (there are very rough imbalances) and indirectly in how they calculated the Power of units (ignoring speed and most abilities, not to mention other nuances). In games where they really work on the balance, they release many patches with adjustments to the characteristics. But this is not the case with Heroes 5.

This is how the balance was built. Offense has too many cool perks, everyone wants offense. Okay, let's reduce the base offense, it's easier and faster. That is, we don't analyze the reasons, we treat the symptoms.

And here is the result of this approach. No one will pump up the base offense more than is required for the perks, thus the effective variability of pumping up the offense is significantly reduced, compared, for example, to defense. Defense can be pumped up without perks at all. They can be pumped up partially with perks. They can be pumped up for the perks (like offense). There are many more options.
Or another situation. The player wants a cool perk, let's say, Retribution. For this, he pumps up the offense by +5. But he doesn't have time to take Retribution, and the opponent attacks. Since the level-up was wasted, the opponent gets a noticeable advantage, just because he was lucky.

Okay, summarizing:
- a decent base for offense is 10/20/30%
- if we want balance, we should bring Battle Balance to level 3 (by the way, we can also leave Retribution as a level 3 perk in this case)
- if we bring Battle Balance to level 3, and leave the base at 5/10/15, then we significantly weaken the offense complex (the question is: is it necessary), to the point that it may not be taken at all