You're approaching the situation from the wrong angle. As a Space Marine player, you see that Blood Angels and Dark Angels and White Scars are similar and they all wear power armor and share many models, so why shouldn't you get to easily take BA/DA exclusive units to be competitive?
However, you should treat these different chapters like what they really are: completely separate Imperium factions. You wouldn't complain about your competitive Ork army not being able to take Heldrakes or Hive Tyrants, so as a non-Dark Angels player, you should not be complaining about not being able to take Deathwing, etc.
The fact that the models and armies are so similar and so close yet so far is gravy, it's something that should be cherished and appreciated for its convenience and compatibility, not as a reason to become spoiled or complacent and to be used as a scapegoat as to why your Space Marine faction should get all the shiny toys available to other Space Marine factions as well.
Also fundamentally it should be noted that while UU exist is just as important to ask what those factions don’t have. To put it one way. Let look at two “Fast” moving Melee Loyalist Marines:
Blood Angels get:
And a several jump pack strategems. I don’t own 9th edition supplement so gonna make a few bold presumptions. Correct me I am wrong:
Descent of Angel is a Redeployment for Jump Packs
Forlon is a Movement for Death Company Jump Pack Units.
Lucifier Engine is something for Baal Predators properly an advance based strategem.
But beyond that: what does BA have for mobility? In their base rules:
+1 Advance & Charge
Their assault marines can take special weapons. So in terms of speed we have:
1) Unique AM Loadouat
2) Jump Packs and related Buffs (Forlorn and Descent)
3) +1 Adv & Charge
UU we have two: Ko’sarro and the the Generic Khan.
Almost every strategem plays into advancing and pewpewing, or fallbacking and fighting. In many ways White Scars are like Harlequins, they have a massive flexibility of both how they engage. And how they engage.
1) Assault Weapon Pew Pew at No Penalty
2) FallBack & Advance then Charge
3) Several more Strategems than BA has for Mobility
So like the question to ask here. While admittedly a Super Cool Biker Squad be awesome (atleast Ko’sarro get his bike back). I don’t want to say its unneeded. But I would rather argue, what would it add to White Scars factional identity?