Why were the internal polls so much more accurate than public polls

[Motorcity]

Both the Biden and Trump capaign internal polls were much better than the any public polls, like the ones used by 538 and RCP

By election day, the Biden campaign knew they were ahead in Georgia but losing Florida. And throughout the campaign they knew Texas was out of reach despite almost every poll showing a tossup. Both campaign knew Michigan would end up close, hence the last minute stop there.

What data did the internal polls have that respected pollsters like NYT/Sierra, Emerson and Public Policy Polling not have?

My guess would be registration data/donor data that belonged to the national parties

[mileslunn]

Both the Biden and Trump capaign internal polls were much better than the any public polls, like the ones used by 538 and RCP

By election day, the Biden campaign knew they were ahead in Georgia but losing Florida. And throughout the campaign they knew Texas was out of reach despite almost every poll showing a tossup. Both campaign knew Michigan would end up close, hence the last minute stop there.

What data did the internal polls have that respected pollsters like NYT/Sierra, Emerson and Public Policy Polling not have?

My guess would be registration data/donor data that belonged to the national parties

That is an issue everywhere not just US.  In 2015 in UK, polls showed Labour and Tories tied but both party's internals knew that was not the case and the surprise Tory majority was not a surprise to either party.  Getting accurate polling is very expensive and most public polls are cheaply done so not as rigorous in methodology as internals.

[Pericles]

Are they really more accurate? It's easy for people to leak and say they had special knowledge to make themselves look good. Biden might just have been playing it safe in Michigan since contesting the tipping point state is more important than contesting the closest state, and he did make a play for Ohio so he might just have had a slightly out of date view of the electoral map. How would we know what he was really thinking?

[DaleCooper]

Are they really more accurate? It's easy for people to leak and say they had special knowledge to make themselves look good. Biden might just have been playing it safe in Michigan since contesting the tipping point state is more important than contesting the closest state, and he did make a play for Ohio so he might just have had a slightly out of date view of the electoral map. How would we know what he was really thinking?

This is true, and we'll never know for sure, but I've been reading (very slowly, I'll add) that book Lucky, and the way they talk about the polls leads me to suspect that it's a well-known and accepted fact within the campaigns that these public polling firms are unreliable.