FHVs are a big factor. Like taxis, they spend lots of time idle (meaning they are single occupant) and circling blocks for fares. It is similar how in some areas, the congestion is simply due to too many people circling for parking. It should be noted that if you, let's say, removed all 100k FHVs tomorrow, we would eventually yet again reach a new equilibrium where congestion will be as bad as it is today, as 100k private veh will simply replace the FHVs. Congestion will never go away and it cannot totally be solved. Instead, the effort should be on keeping the streets safe and usable by people.