That's an easy question... A bit like why does (*) lick his (**)? :-)
Because they are cheap and easy to get in case of your question.
That they aren't optimal for colour reproduction - that's a problem that only a few people understand and care about...
What light are you using? Or maybe you're just painting in the sunlight and do other jobs after dark?
Good luck with your modelling projects and have a nice day!
Paweł