I think you might’ve misread this table. If you count up the number of bear markets in the table and then look at which of those periods also had a recession, the number of bear markets that coincided with a recession is actually ~50%. Further, the table says they count a S&P500 decline of 19% as a bear market, which by our arbitrary standards, isn’t. So if you take those 19% drops out a recession actually coincides with a “true” bear market about ~65% of the time according to this table. I fail to see how you got 20%. Maybe you were looking at the length of recovery which it says on average is 20 months?
I was commenting about how historically bear markets start around the 20% mark to the original comment regardless of a recession. This is consistent with many books on the subject too and thus isn’t just “arbitrary”. It has actual data going back hundreds of years
Pretty much every definition mankind uses is purely arbitrary, but that doesn't mean the concepts are not useful. The only things that are not purely arbitrary are extremely rare, such as a few concepts in math (primeness, for example) or physics (number of fundamental forces, universal constants, etc.).