Because when you come back to England and apply for a job, you end up with a lot of rejection letters or no response at all. This is because most airlines have no burning need for people with 300-400 hours. Of those that do, it is usually as a result of a cadet programme that is contracted to or affilated with one of these Integrated training providers.
With a couple of thousand hours of verifiable turboprop / jet, experience to your credit, you may well satisfy the basic qualification criteria for most airlines direct entry protocols, but a couple of hundred hours of Cessna time from the cheapest overseas "puppy farm" to be found, isn't likely to excite much interest at this level. That isn't to say that this method is unsuitable for an airline career at some point, but for airlines with cadet programmes, they are not usually interested in somebody rolling up at the door with minimum experience aquired, that they (the airlines) haven't themselves had an involvement with.