Maybe because he only has to beat one person. The Dem nominee. He has a straightforward path the 2020 election.
Meanwhile, every Dem nominee has to beat every other Dem nominee before they get a shot at Trump. They all have a tougher path to the 2020 election. That's why they have longer odds. That's it.
The odds you present are not head-to-head odds.
Lessoneth overeth.