Is it really a matter of skill? I've never really considered tipping by 15% anyway, but shift-right, halve, and add back to get 15% doesn't seem fundamentally more difficult than shift-right and double. Just more tedious. Anyway, nowadays everybody has a calculator on them at all times so this couldn't be the reason.

Just move the decimal one place and add half of that value to itself. 15%. Ez

It amazes me that people can't calculate this, even roughly. Its a tip, it doesn't need to be exact

20% is one less step and doesn't require addition, although I'd call it more 'aversion to tedium' than a lack of talent.

I also feel like it may be a bit of a stretch to call division by 2 tedious, but I also do math on a regular basis so I may be biased

Invest in some precalculated values and $6.66 @ 15% earns $1. And if it's all about ease, what could be easier than "metric" 10% ?

/s because the ship has sailed.