Otherwise, people could just vote whatever they wanted to happen.
Very unlikely, unfortunately. Or perhaps luckily, given what some people want!
The problem is you can't vote for a single thing. Well, you can, but doing so drags along several pages full of other manifesto commitments, many of which you might prefer to vote against in preference to the thing you want to vote for.
There is also the small problem of not being able to vote for something that isn't on offer. If, for instance, neither of the main parties in the UK say they will ban excessive profits from motorway service stations then no-one will be able to vote for that regardless of who they align behind.