Many mature democracies, whether their system is parliamentary or presidential in character, do not play the game of fiddling around with election dates to suit the convenience of the incumbent government.
Britain is not alone in having this system, partly because many other countries have copied our system, partly because some countries which regularly experience difficulty in building a coalition government have faced the genuine need for an election to clear a parliamentary logjam.
I don't believe there is any one perfect system which is right for all countries in all circumstances. But I note that the USA, France, Germany, the European parliament, the devolved parliaments and assemblies in this country, and all our local councils manage perfectly well with normally fixed terms for most of their elections. This need not be inconsistent with arrangements to change the date of elections which would normally be fixed in genuinely special circumstances, such as war, disease epidemic, or if no viable government can be formed in a particular parliament.
For example, during the Foot and Mouth outbreak in 2001, parliament passed a special bill to move the fixed term County Council elections which would normally have taken place in May that year back a month (co-inciding with the General Election). Germany normally has fixed parliamentary terms, but there is a special procedure, which is not often used, in which a special election can be called following a "constructive vote of no confidence".
I was interested to see the website "Fixed Term" to campaign for fixed term parliaments, which you can read about here.
I have a genuinely open mind on this, but my immediate reaction is that they make a strong case. I would be interested to read the comments any readers here might like to make on the subject.