I think the issue is there is a lot of hidden information in the bike industry and the industry doesn't want consumers to realise a huge percentage of bikes are effectively coming from the same manufacturer and are similar quality or that many brands are not actually manufacturers at all and simply buying production from the same big Chinese manufacturers.
I feel you're both right and wrong here. I imagine there are 2-3 very large manufacturers, it's the nature of modern industry, so I agree a large percentage of frames are from the same source(s). Are the brands desperate to hide this? No, I don't think so. While a brand may not want talk about where or by who it's frames are manufactured I can't recall ever seeing a major brand shout out about having it's own manufacturing capability. If a brand did have it's own production facility this would be a major plus and one which a brand would shout from the rooftops.
I've never been excited by Giant or even considered their range, similarly with Trek, Specialised, Scott, Canyon etc. which is either my ignorance meaning I've missed a lot of good bikes or a failure by the brands to excite me. I'm not sure which but lean towards the latter.
I've owned and loved a Monoc, Dolans and currently ride a Cervelo and Kinesis both of which are great bikes. The Kinesis ATRV3 ticks every box for gravel/winter/touring and probably good for another 10 years while my Cervelo C3, now six years old, is the best bike I've owned. My love of the Cervelo is such rather than consider a new bike, I worry I would find anything as good, I recently spent £800 on upgrades/full replacements. Dolan, Cervelo, Kinesis and Cannondale would be my first brands to consider. I've seen some very nice Reilly and Mason bikes and would look at these.
Does it worry me these bikes could all have come out of the same factory? Not in the slightest as it's the design and build I can buy which interests me. I must confess to being slightly disappointed two weeks after getting the Kinesis when a friend turned up on his new Reilly with an almost identical frame. A few weeks later two other friends turned out on new Dolans with almost identical frames! I was surprised by this as was my LBS when I told them.
I think it's interesting how people are grabbed by looks. Before Christmas I turned up to a club ride with a buddy on my Kinesis, she on her identical Dolan to find another rider turn out on a brand new Reilly with a great paint job. After a lot of ooohing and aaahing by my buddy over said Reilly I quietly mentioned "you know it's the same Ti frame we are both riding but painted?"
It would be fantastic to see the rise of a quality British bike brand manufactured in the UK. A pipe dream I guess.