When a whole host of champions are unable to outdamage certain champions' lifesteal while itemizing for damage,
It's pretty much impossible to do so little damage life-steal completely negates all damage if you itemize for damage.
If you're pure-tank Braum and you bought a Sunfire Cape for your only 'damage' item, sure - life steal is going to erase the damage you deal.
But the majority of champions in LoL have a QWER combo with bases on-par with Ekko, getting 1000 damage from bases alone when they hit level 18, where the typical HP of any given life-steal heavy squish is under 2000.
In games where a life-steal carry is a little behind, it's also not uncommon for certain tanks like Amumu or Malphite to OHKO them with their initiation combo while not building much in the way of damage.
Also: while Thornmail isn't good in many situations, it's exceptional at completely negating life-steal from having any effect on it's wearer if you're playing any champion that can't itemize for damage. That's the niche thornmail fills.