You say that your router comes from Europe and is metric. I have European routers (Bosch, Elu and deWalt) and they all have both metric and Imperial collets available. Same applies for other European brands, such as Felisatti. Freud, Metabo, Einheil, Scheer, etc. If you post the make and model of your router I'm certain that someone here will be able to help you source an Imperial collet
I agree with you about the availability of metric shank cutters - in mainland Europe 6mm, 8mm and 12mm are the standard sizes for portable routers with few (rare) places selling 1/4in or 1/2in shank cutters (the UK and Ireland standard sizes), but you can get some
over the counter metric shank cutters - try a Trend or Titman stockist because both do a limited range of straight cutters (and some others) with metric shanks. For mail order I tend to go to Wealden Tool because over many years I've found their quality and service second to none. Good prices, too. They have a range of 8mm shank straight cutters
which might be of interest to you.
As an aside I think the 8mm shank size in the UK goes back to Elu days - they always suppliedm (or made available) their smaller routers with 8mm collets. An 8mm shank is much stronger than a 1/4in one (60% more cross section area) which means that in the hands of ham-fisted tradesmen like me they are less likely to break!
Profile cutters often take a much lighter cut, so there was less urgency to have 8mm shanks on them, and that's where things seem to have stuck