Buying power.
I don't know exact figures so note that these are plucked form the air.
Specialized, Trek and Giant are by far the biggest manufacturers of bikes as we know it, ride, and discuss here. Their buying power is MASSIVE as they have a) volume and b) range. So they're buying lots of everything. From entry to top end, in all cycling genres.
When they go to SRAM and buy thousands and thousands and thousand of groupsets (each!), they pay less than the brands buying a thousand. They will also get priority in the supply chain, will get first word on new developments (quite often develop it with component brands) and have racing teams sponsored by component manufacturers - both for dev and marketing reach.
Simply put, they pay SIGNIFICANTLY less for every part on their bike than smaller brands.
To add to the cost for smaller brands...
R&D quite often transfers. Not always 100%, but once an engineer learns a trick it's much easier to adapt than to work it out from scratch. So carbon tech transfers between bikes. Shock tech and research transfers between bikes. When you have a bike in each category on the face of the earth, the tech transfer scales at a ridiculous level. Smaller brands have to learn the lessons the hard way and usually on one or two models each in MTB and road. Smaller brands do not always have their in-house carbon / mold / manufacturing experts. Yes, they can design bikes in-house, but those then need to get tested somewhere else. All of this costs money, and usually comes with a long queue.
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Big brands can easily offer better value for money. Smaller brands offering value for money cuts deep.
