My only guess is that 6.5 has become so popular that it's just a marketing thing.
I can say for certain, “6.5” was not popular when the 123’s were developed for the 6.5 Grendel.
The choice to use 123 grn in Grendel ammo is likely much more dependent upon the long standing market of 123grn 7.62x39 ammo, the parent cartridge to the 6.5 Grendel, AND as a means of segregating the short ogive and low expansion velocity 123’s to be used in STANAG length magazines rather than the longer ogive 120’s which could be used in other, more powerful 6.5/.264” cartridges.
I know of no other caliber where this is true.
I think you’re not paying very close attention, lots of makers offer multiple bullets in the same caliber from the same model line.
All manufacturers offer multiple bullet weights within a given bullet line for a given caliber, and in many cases, we end up with multiple bullets of the same or similar weight within the same model line which are designed for different expansion velocities, or they’ll offer new bullets at a given weight and retain the old bullet in the model line - the 6.5mm bore diameter isn’t unique in this.
A non-exhaustive list of examples:
Hornady makes 6mm bullets in 87, 90, 95, 100, 103, 105, 108, 109, 110, including the same ELD line bullets in 90, 103, 108, and 109grn. They make 22cal V-Max bullets in 35, 40, 50, 53, 55, and 60 grain. They make their Varmint bullet in multiple weights as well. They make their 22 cal ELD’s in 73, 75, 80, 88, and 90 grains. Hornady makes 7mm ELD and ELD-x bullets in 150, 162, 175, and 180 grains. They make multiple 30 cal 150grn SST bullets designed for different expansion velocities… you can pick any of a dozen calibers and find bullet weights which nearly overlap. They also make XTP and XTP-Mag bullets, and XTP-Flat Point bullets which share the same weights but use different designed expansion velocity windows - coincidentally having the same weight, but performing very differently.
Sierra makes 22 cal Matchkings in 52, 53, 69, 77, 80, and 90 grains. They make 6mm SMK’s in 97, 107, and 110. They also make 7mm SMK’s in 168, 175, 180, and 183grn. In 30cal, they offer SMK’s in 150, 155, 168, 169, 175, 200, 210, 220, and 230…. They also make Tipped Matchkings in weights which overlap these weights offered in SMK’s, some of which cannot be used in the same applications (for example, the 77 TMK is longer than the 77SMK and can be a problem in AR mags).
Berger makes 6mm Hybrids in 105 and 109, and makes their VLD’s in 105, 110, and 115. Berger even makes two 200grn 30 cal Hybrids, one just slightly more aerodynamic than another.
Nosler offers their 30 cal Ballistic Tip Hunting bullet in 165 and 168.