They're not.
Obviously there's a lot of different types of steels, then there's quality control, custom tubes/frame parts, finish of the frame etc, just because one steel frame made in a shed might cost £200 doesn't mean all steel frames are in that same bracket. You can get a cheap alloy frame from a factory in Taiwan for peanuts, that would be cheaper than a good quality steel frame so steel isn't always cheaper.
Most custom Ti frames will be upwards of £1000 barr one company which you'll have trouble getting a frame off any time soon.