But knowledge does not come in two grades, "scientific" and "useless". Anecdotes do count as Bayesian evidence, they are just weak Bayesian evidence. And well designed scientific studies constitute stronger Bayesian evidence then poorly designed studies. There's a continuum for knowledge quality.

Methodologically, each self-experiment is typically much more poorly run than the kinds of trials we try to discuss here (RCTs), so each self-experiment represents less than n=1 of data. The RCTs usually have at least a few dozen and ideally hundreds or thousands of subjects, either singly or pooled for meta-analysis. So a single such meta-analysis represents thousands of subjects times the fractional quality of a self-experiment, leading to the conclusion that one self-experiment is worth somewhere less than one-hundredths to one-thousandths of any comparable RCT.

The human mind doesn't do 64-bit floating weights. It doesn't even do shorts.