Because there's basically no design downside to having a higher power rating than needed, aside from BOM cost. If you're ordering a bunch to have on hand, you should just order the highest power rating you're likely to need in that size.
For me, that means that my 0402s are all 1/16W, 0805 are 1/8W, 1206 1/4W, etc. And all of my through-hole resistors are 1/4, because the wire stock plays well with breadboards better.
There are probably 1/4W 0402s out there, but that's definitely a specialty piece. I'm seeing 16 cents a resistor/each for a 1 MOhm 1/4W 0402, which is about 4 times what I'd expect to pay for a 1/16W of the same resistance and package.
I'd be surprised to find a 1/4W 0402, you'd just about melt the solder off. Yageo claims this one is good to 3W, do you think it glows cherry red? What trace width and pad geometry do you need to push 3W into a 0.0025 ohm resistor?
But to your point, Digikey has >70,000 0402s in 1/16W. There are 900 rated for 0.05W, and they're all exotic high-frequency/low temp coefficient/high-precision specialty parts.
