For some reason, many developing companies dont have demos to try their game before buy ...
I don't know why most do it, but I seem to recall seeing an argument put forth by Extra Credits that putting out a demo is somewhat risky for the developer--even (and perhaps especially) if the game is excellent.
In short, and if I recall correctly, the argument is thus:
The game itself might be roughly grouped into three categories: bad, okay, and good. (One could use a finer division, but I don't think that doing so changes much, and fewer groups keeps the argument simpler.) A demo likewise may be fitted into the same three categories. However, the category into which a demo falls doesn't necessarily match that of its parent game--it could show off only the best of an otherwise poor game, or the worst of an otherwise excellent game; something might shine in a short demo, but become tiresome in the full game; the developer might stumble over some element that's fixed or not present in the full game. In short, a demo is a nearly-separate product from the game.
For a poor game, this may be advantageous: an okay or good demo may boost its sales. An good game, on the other hand, might have its sales pulled down by an okay or poor demo--only a good demo would be likely to be advantageous.
On top of that, there's the element of word-of-mouth: a particularly poor or good game will likely develop some degree of reputation even without a demo. This is perhaps particularly true these days, with video reviews and Let's Plays providing a means of discovering and informing one's thoughts on new games.
Finally, producing a demo will likely call for additional development time on top of the production of the game itself--even if the demo is just a few levels from the main game, there may be additional logic to handle the end of the demo (such as "end of demo" screens, etc.), and one way or another it's likely an additional build to make and test.
That said, I do suspect that crowdfunded games might benefit from demos as a means to show that the game is under production, and looks promising (presuming that it is and does, respectively).