There are diminishing returns on bonuses to attack, and a mid- to high-level PC is likely to meet them even without Weapon Focus. Monster AC doesn't scale as quickly as bonuses to hit do, so eventually a Fighter will be trivially hitting the monsters, causing Weapon Focus to lose its efficacy.

Power Attack is the easiest comparison, and while Power Attack and Weapon Focus are comparable to each other for at least the early to mid levels, the subsequent feat chains are not: Weapon Focus -> Weapon Specialization -> Greater Weapon Specialization pales in comparison to Power Attack -> Improved Bull Rush -> Shock Trooper.

Outside of a raw damage comparison, feats like Improved Trip and Combat Reflexes provide utility and control tools that are much more powerful than a half a point of extra average damage, and something like Improved Initiative can mean the difference between getting to act or just getting crowd-controlled in the first round.

Honestly, Weapon Focus is fine at low levels. It compares pretty well to Power Attack mathematically, better against high AC enemies and worse against low AC ones, but the high AC enemies are often the more difficult fights. But the Weapon Specialization has pretty minimal impact compared to the things Power Attack opens up, like Shock Trooper or Leap Attack.¹

Lastly, Weapon Focus locks a PC into a particular type of weapon, which can sometimes be a problem. If you're delving in a dungeon an uncover a sweet +2 warhammer of skullcrushing but you have Weapon Focus (Longsword), that's going to feel bad. If you took it for an executioner's mace, like you're considering, the chances that you find an upgrade in a dungeon or even at an weapons shop are slim to none, so you're reliant on getting a custom made weapon, which isn't actually trivial in many campaigns. (Weapon Focus for a natural weapon dodges many of these issues, as does the Warblade class feature Weapon Aptitude.)

Weapon Focus does lead to plenty of neat prestige classes, like the Pious Templar or Divine Crusader, and it's great for NPCs because it's easy to just bake into a stat block, as opposed to a "better" feat like Combat Reflexes or Power Attack. And if you're just taking pure levels of Fighter, you'll have so many feats that you might as well take Weapon Focus. But for most characters, there are simply better things to be doing with feat slots.

1: Though it's perhaps worth noting that, as written, Leap Attack doesn't do anything at all because the author forgot how Power Attack works and also how charging works, so I suppose in that light Weapon Specialization is much better. But it's pretty clear how Leap Attack was intended to work, and it's stronger than Weapon Specialization.

There are diminishing returns on bonuses to attack, and a mid- to high-level PC is likely to meet them even without Weapon Focus. Monster AC doesn't scale as quickly as bonuses to hit do, so eventually a Fighter will be trivially hitting the monsters, causing Weapon Focus to lose its efficacy.

Power Attack is the easiest comparison, and while Power Attack and Weapon Focus are comparable to each other for at least the early to mid levels, the subsequent feat chains are not: Weapon Focus -> Weapon Specialization -> Greater Weapon Specialization pales in comparison to Power Attack -> Improved Bull Rush -> Shock Trooper.

Outside of a raw damage comparison, feats like Improved Trip and Combat Reflexes provide utility and control tools that are much more powerful than a half a point of extra average damage, and something like Improved Initiative can mean the difference between getting to act or just getting crowd-controlled in the first round.

Honestly, Weapon Focus is fine at low levels. It compares pretty well to Power Attack mathematically, better against high AC enemies and worse against low AC ones, but the high AC enemies are often the more difficult fights. But the Weapon Specialization has pretty minimal impact compared to the things Power Attack opens up, like Shock Trooper or Leap Attack.¹

Lastly, Weapon Focus locks a PC into a particular type of weapon, which can sometimes be a problem. If you're delving in a dungeon an uncover a sweet +2 warhammer of skullcrushing but you have Weapon Focus (Longsword), that's going to feel bad. If you took it for an executioner's mace, like you're considering, the chances that you find an upgrade in a dungeon or even at an weapons shop are slim to none, so you're reliant on getting a custom made weapon, which isn't actually trivial in many campaigns. (Weapon Focus for a natural weapon dodges many of these issues, as does the Warblade class feature Weapon Aptitude.)

Weapon Focus does lead to plenty of neat prestige classes, like the Pious Templar or Divine Crusader, and it's great for NPCs because it's easy to just bake into a stat block, as opposed to a "better" feat like Combat Reflexes or Power Attack. But for most characters, there are simply better things to be doing with feat slots.

1: Though it's perhaps worth noting that, as written, Leap Attack doesn't do anything at all because the author forgot how Power Attack works and also how charging works, so I suppose in that light Weapon Specialization is much better. But it's pretty clear how Leap Attack was intended to work, and it's stronger than Weapon Specialization.