I think theres multiple reasons why online is harder
1
Profitable players mass multitable online whereas the fish generally just play 1 or 2 tables, which makes the ratio of strong players on average tables much higher than live where everyone can only play 1 table.
2
People who aern't really serious players just go for the social experience of playing live in the casino. I think most people would agree its more fun playing live, so it attracts more casual players.
3
New Players online can play in micros as low as 1NL. In
casinos its hard to find lower than 100NL so brand new players are forced to play at this high.
4
Probably the ratio of strong players to weak is higher in young people. I'm not being ageist but many bright young people got into it after the Moneymaker boom, who wouldn't have before. The ratio of young people is going to be higher online because young people use computers much more on average.
I myself watched a live cash game for a while at 200NL in my local casino, and was surprised at how bad a lot of the play was. Most players were just playing way too many
hands, and there was just so much open limping, and 4-6 players commonly seeing a flop. One guy just went all in preflop with AJ about 100BB. The level of play at 50NL online which I sometimes play is certainly way higher.