Why are most bicycle gears designed to have so much overlap For bikes with a front derailleur, I've noticed that the gearing ratios of most bicycles are not linear, and there is an odd "overlap" between those power ratios as you switch from one front gear to the other.  

OK, it's hard to explain, but this chart shows what I mean:



This is just an example and actual ratios can vary, but this illustrates some extremely common problems:


The ratios aren't spaced out consistently
To go down through all the gears from highest to lowest, you will find yourself switching front gears several times back and forth while simultaneously switching the rear.
Many of these gears are so close as to be redundant.


So I'm just curious: why are the gears set up in this way?
Having an overlap between the ratio ranges available with each chainring means you don't have to use your front derailleur as much. Based on your understanding and expectations of the terrain and conditions ahead, you choose a chainring which gives you access to a useful set of harder and easier ratios.

The cassette ratios are spaced to achieve a vaguely consistent %-difference between adjacent gears, with the limits that you can't have fractions of a tooth and for smooth shifting the derailleur's upper pulley has a minimum and maximum distance from the teeth, so the derailleur's geometry needs to move that pulley through an arc that echoes the arc of the cassette.

Having ratios closer together means when accelerating you don't have to "spin out" one gear to avoid getting "bogged down" in the next harder gear, and when climbing you don't lose momentum with a sudden loss of resistance on each change down a gear.

Alternative 1: Internal hub gears

e.g Rohloff 14 speed, Shimano Alfine &amp; Nexave 11,8 or 7 speeds.
Low maintenance but heavy and make removing and fitting your rear wheel more difficult.

Alternative 2: Single-chainring MTB setup with a wide-ratio cassette

10 and 11 speed systems are available with smallest sprocket 9,10 or 11 teeth and largest sprocket between 40 and 50 teeth, for an overall ratio difference from hardest to easiest ratio of 4:1 - 5:1. The derailleur needs to be designed for the steeper cassette angle, of you can use a hanger extender (e.g the GoatLink) to move it down so it clears the largest sprocket, but in the hardest gears the upper pulley may be too far from the teeth to shift nicely.