Cost/benefit matters. Concentration is a high cost but 5e is designed so the benefit of that spell are not needed and failure on a roll doesn’t really matter. Sure it looks good on pwper, but so does the mpg results of hypermiling & both are beyond the point of being meaningful benefits for the cost... That's why it's listed.
Yes, that is why I described the benefit.
On 3 rounds on 3 melee PCs, it comes to more than a melee PC's worth of damage if they had 100% accuracy, if they do nothing to optimize having it.
If they do GWM/SS because they have it and aren't idiots, the benefit will be even larger.
Concentration is there, sure. Bless is a slot-efficient way to turn level 1 spell slots plus concentration into combat power. Most better concentration spells are level 3 or above.
Aura of Vitality -- awesome! Level 3.
Spirit Guardians -- awesome! Level 3.
Haste -- takes as long as Bless to generate as much offensive power, and level 3. Main benefit is speed boost really.
Faerie Fire - requires a save, or does nothing.
Zephyr Strike -- mobility spell with a small damage finisher. Not really competing.
Improved Invisibility -- awesome! Level 3. Accuracy boost is larger, but applies to 1 PC. So roughly similar yield.
Heat Metal -- Requires enemy to have armor, level 2.
Fly -- awesome mobility! Level 3.
Hypnotic Pattern -- awesome crowd control! Level 3.
Polymorph -- unless you are a sorcerer, how do you have both spells? Anyhow, awesome! Level 4.
Bless is a 1st level slot. At low levels, it is what you have. At high levels, the slot cost is nearly free, while the competition slots aren't until late T3/T4.