Too difficult to do - particularly as the taxes change from country to country; surcharges change from airline to airline - it's a lot more than the base legs and base fares.
For example, Sydney's Kingsford Smith International Airport has a A$4.30 noise tax, which other Australian airports either do not charge or charge different prices for, some countries include departure tax in the ticketing (eg. Australia), others do not, and have different taxes at each airport (eg. New Zealand) and the website would have to be able to advise on all of these things; AND
be 100% accurate and always updated.
And not enough demand - most people would prefer to use a travel agent to get everything done, particularly for the more complicated itineraries.
If it's a simple point to point job, it can be done as in some LCCs, and a lot of people do not mind doing a simple o/d return itinerary; but as the Star Alliance fly to 842 airports in 152 countries - that's a massive booking system that would be required! And now with two Chinese airlines coming on board, it will be an even larger system.
SkyTeam and OneWorld, although smaller, have similar problems.
If demand eventually arises - i.e. a lot of people want to dispense with travel agents and take it all upon themselves to organise it - and have no-one but themselves to blame if things go wrong and not being able to claim compensation for it... then maybe it will happen.
Meanwhile, I'm not a travel agent, but there is a reason they're around, and why there are special courses in how to navigate booking systems.
Apart from other things, all those countries have different entry requirements and formalities, which people need advising on - and all that information would also have to be put into the system. Again, a lot of LCCs can do this because they fly to a limited selection of countries OR
the vast majority of their passengers (eg in + between Western European countries) have visa-free entry; but for the magnitude of 842 airports with different tax structures, and 152 countries entry formalities - that's enough on its own really to say - by all means use the round world calculator to make a list of the dates and routes you want, but leave the rest to the travel agent, who should also advise on visa regulations.