Now, why exactly is that?
Eurocontrol would say they are only implementing each country's routing requirements. So this is where the trouble starts. Most N European countries, in their endless quest for gold plating and job creation / job protection issue a totally bizzare set of routing restrictions for their airspace.
These are in turn the result of haggling between mostly the military who like to hang onto various chunks of airspace, knowing that if they let go of it they will lose it for good, and the national CAA.
Eurocontrol's software boffins (to whom "usability" is a totally alien concept) then implement that in their validation software.
However it is obvious that Eurocontrol could have easily provided a route generation facility on their website. They have always had this in-house, and announced plans as early as 2005 to open this up.
What happened then is not public but my info from the inside of Eurocontrol is that the expensive bizjet flight support services (Jeppesen, etc) made a massive fuss. Their business hangs on the "black art" of IFR route development and flight plan filing, and the rest of the stuff they do for their money (overflight permits, etc) can be done quite easily by others. What you do get is an integrated service but take out the IFR route planning and the black art bit is now gone.
So Eurocontrol backtracked on their plans. At a 2008 conference (which I was at) this was put to a Mr Hendrix (a senior ex-ATCO working in there, who was heading the event) and he gave a pompous and totally daft answer that it would require too much computer power
Later, in 2008, a chap called Christof Edel developed a program called Autoplan which would do a maze routing through the airway database, present it to the Eurocontrol validation website, and iteratively modify it until the error messages all went away. It worked brilliantly. Eurocontrol tried to block it by changing their validation portal, changing it to HTTPS and a few days later changing it to a Flash movie site, and Christof who also had other priorities then gave up the chase.
But the incestuous relationship between Eurocontrol and whoever stopped them offering routing was blown for good. Around 2009, other software sprung up which used a "more approved" (i.e. paid for) validation facility. Flightplanpro is the main one, whose functionality is also available via the Rocketroute website. Eurocontrol also then started offering a "route suggest" facility; initially this was crap, with overheads of 100% in some cases, but they kept improving it and today it is pretty good and works most of the time. This feature is accessible via EuroFPL, via Rocketroute, via Flightplanpro, and I guess via other services. I haven't used Homebriefing.com for a long time but I guess they now offer it too.
So, today you can get a valid route which is pretty efficient (5-15% overhead over GC is normal in Europe in the lower airspace system) about 95% of the time. The other 5% you have to manually hack it with some DCTs, but this is getting rarer all the time. I use Flightplanpro for everything and it is at least a year since it has not been able to do a workable route.
All in all, things are much better than they were say 5 years ago. IFR flight planning is now a piece of cake, usually.
The funniest thing is that ATC do not actually operate most of the routing rules which their own country has concocted. They treat controlled airspace as their tactical playground and once you are airborne they couldn't care less about standard routes, semicircular levels, etc. In reality you sit there, FL150 or whatever, on autopilot, and just keep asking for DCT shortcuts the whole way
The original standard route massive job creation scheme is almost totally wasted.
Some countries are better. France for example seems to have very few rules, and you can file routes all over France, in their FL065-FL195 Class E/D airspace, and most of the routes are the same in both directions. This is the closest to the US model.