Why don't phone GPS maps give scale?

Developing for the smallest format smart phone, it would use valuable real estate. Plus it would have to be “fluid” in its rescaling, as zooming in and out is not to set ratios, but is on a continuum. If it maintained it’s relationship with the map, it would have to grow and shrink as you zoom in and out.

Ordnance Survey online maps for the UK give the scale. As zenvelo says the scale adjusts as you zoom in and out. It works fine on my mobile phone.

Google Maps has a scale, at least on the Android version, though it disappears 2–3 seconds after pinch-zooming.

As for not taking up much space, that may be true for a “phablet”, but on a 4.3” screen like my Moto E, it takes up a fair bit of space, and some smartphones (like iPhones before the iPhone 6) are so tiny that it’d take up nearly the entire bottom quarter of the screen. Any scale that would be legible to most people takes up about ¼” of vertical space and at least ½” horizontally, and not all phones can really spare that.

I guess I haven’t seen Android Google Map in-action!
I was imagining a simplified scale—just a thin line of a fixed length, its measurements changing as someone zooms in/out—from “10 ft” to “1 mile” to “5 miles” etc. My phone’s map program already takes up screen space with an “x yds from ground” bubble that isn’t terribly useful, so I didn’t imagine a tiny little scale line being much of an issue…. and since the GPS programs already have to know the distance of things—“right turn in 500 feet”—I didn’t think calculating scale would be much computational effort. But I can see how implementing a uniform scale would be problematic… the length of line would need to change depending on the phone’s screen size—but it can’t simply be a proportion, like ¼ the width of the screen, or else it would be very small for some that maybe are narrow but tall; but then a fixed length would be problematic too, since it would show up smaller on some screens and larger on others—and for scale, too small and too large makes it less functional. Also, my phone’s screen is on the larger end (2½” x 4”) so I’m less aware of real estate.

It takes up about ⅓ of the bottom edge of my screen, though it does vary in length as you change zoom levels. It doesn’t vary too much though since every halving or doubling of zoom level will halve/double the units displayed.

The difficulty is not computing the scale, but finding a way to display it without cluttering the screen too much. A little math shows that your phone is right around 4.6”, so you do have a bit more screen than some people. And when I tried it on my wife’s old phone, a Droid Ultra with a 5.0” screen, it does indeed take up a smaller portion of the screen.

FYI, the main limits on how small a computer can be these days are strictly human elements. Smartphones are about as small as most people can use for anything comfortable for typing on a QWERTY keyboard, though a T9 input could get it down to wrist-watch size… if the user has fingernails. My old Casio Databank watch was like that. Displays could get a bit smaller since objects appear larger close-up, so something like Google Glass is about the lower limit until we find a way to put a display into a contact lens. But for most purposes, the smartphone is the smallest general-purpose computer possible so long as we still interact with computers with our eyes and fingers.