Why not? Try running a high ohm coil at a high wattage and let me know how that works out for you. It sucks. The voltage increases dramatically at a higher ohm and results in a burnt taste (for me at least). It is intolerable.
A 2 ohm coil @ 20 watts = 6.3 volts. Since no battery can do 6.3 volts, the "regulator" in the mod will throw more current at it to compensate, so you're really no better off.
I have a kayfun in front of me with a 1.3 ohm build. I usually sit at 12 watts or so. I just cranked it up to 20 watts, and even with a 1.3 ohm build, it's too hot. If I build sub-ohm I can hit 20 watts all day with no burning.
A lot of it has to do with the coil itself, the surface area, heat flux, etc. This is why a higher ohm build at (say) 30 watts just doesn't taste the same as a lower ohm build at the same wattage. Instead of going into more detail, I recommend you read the thread I am linking below. Read all of
Rusirius' posts as he explains it very well. (especially posts #4 and #10).
Link:
http://www.e-cigarette-forum.com/fo...gh-wattage-vaping-resistance-differences.html