Well, it doesn't, because as I mentioned there's the 205. People may choose to take a taxi for convenience, though I think based on the layout of both stations you might actually walk less far if you use the 205 westbound (eastbound it'd be a lot further with two crosses of the road).
Euston Square westbound really needs a lift, then there'd not be a problem. (People seem to have an issue with the street level walk to that station, and it is a bit silly that there's no entrance at the other end given that the station runs from the present entrance to more or less directly outside Euston, but you actually walk about the same to get to Euston Square from Euston as you do to get to the Northern Line Bank branch or the Victoria line, you just walk that underground).