List of output currents in test conditions in CMOS buffer's datasheets
CMOS buffers have a specific ‘on’ resistance, Rds(on). The on resistance will vary with process and temperature.

But rather than state Rds(on) as a range, they characterize the buffers to meet specified Voh/Vol for the I/O standard they’re designed to drive. I suppose that saves you the trouble of figuring that out from Rds(on).

As to why those particular values, they’re powers of 2. ASIC libraries are the same way. It makes sense when you consider how pads are configured, as transistor regions that get turned on in chunks to achieve the desired drive strength.