I suppose I'm unusual then
I've bought stuff from European online retailers in the past, and still do.
I saved a small fortune on quality bathroom fittings (because the Germans know how to do engineering and what they consider ordinary fittings are imported to the UK and sold as luxury lifestyle accessories at absurd prices).
I've also bought musical instruments from Europe, because they were cheaper.
I remember in the mid 90s chatting to some European friends who had ended up buying IT stuff from Gateway in Ireland because it was cheaper.
I regularly buy CDs from independent labels and bands on the Continent, mostly in Scandinavia.
I would imagine the problems are largely down to all the obvious stuff: language barriers, market sizes, regulatory uncertainties, taxes, and shipping costs.
The UK is a big market, so retailers set up to serve the UK will benefit from economies of scale. The same isn't true of, say, Portugal, and the cost of legal and translation work that would be needed for a Portuguese to serve the German or English markets probably means only bigger retailers would bother. VAT is not uniform across the EU, so a Danish company
VAT is generally quite a lot higher on the continent, so a Danish company (25%) might find it hard to compete on price in the UK, or would have to ask the user for their country of origin before displaying the price or somesuch.