Maybe because you don't meditate for longer than an hour and sitting for less than an hour is unlikely to give you blood clots.
GP about blood clots. Likelihood of clotting also depends on sympathetic arousal:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2629605/
So stimulation arising from eating food, drinking coffee, work stress, etc, whilst being seated would be a double whammy.