Our eyes are much more sensitive to Blue/Violet, and less sensitive to green. There's a bunch of maths behind the perceived responses of our eyes.
If you're interested you can calculate perceived differences in the LCh colorspace using something like CIEDE2000.(https://en.wikipedia.org/wiki/Color_difference#CIEDE2000)
A Rainbow with equal DE between colors would probably look better than this one with equal DL.
The main purpose of ΔE is specifying error tolerances for specified colors / measuring small differences between nearby colors, so that e.g. if you hire someone to paint a car or print a magazine you can check that their output matches expectation sufficiently. For colors that are significantly different ΔE isn't that helpful a practical tool.
Hm? It's the exact opposite. That's why the full-intensity RGB green looks much brighter than the full-intensity blue. To convert RGB to pure luminance (gray), you do something like 0.3 * R + 0.6 * G + 0.1 * B, meaning green contributes six times as much as blue.