Why are aerial taxis multirotored, instead of a single rotor?
There are a few main reasons most electric air taxi concepts have multiple rotors.

1) Mechanical simplicity.  Helicopters are controlled by varying the angle of attack of the blades as the rotate, and the mechanical mechanisms to do this (swashplates) are complex and expensive to build and maintain. The thinking is that with electric motors, you can stabilize the vehicle using differential thrust and eliminate the complexity of those systems.  This has implications for vehicle cost.

2) Noise.  One of the main reasons there aren't any helicopter airlines around now (and there used to be) is that helicopters are very noisy.  Noise in rotorcraft is related to a lot of things, but one of the main drivers is blade tip speed.  With more, smaller rotors, you can have lower blade tip speeds for the same disk area, thereby reducing noise.  Whether this has the potential to reduce noise enough to make a significant difference has yet to be conclusively demonstrated, mostly because there's no clear definition of "enough noise reduction".

3) Safety.  Another argument commonly made for eVTOL air taxis is more motors = more safety, because if one motor fails you have lots of others to keep you in the air.  The validity of this argument depends on how much redundancy you have in the rest of your powertrain, but it principle the opportunity for more redundancy offers a pathway to improved reliability.  There are new failure modes introduced by this multitude of rotors and in general these vehicles have no autorotation capability, making a total power failure much higher consequence; all of which needs to be taken into account.  

4) It looks cool There's probably more to this argument than it gets credit for.  Most of the investment and momentum behind the air taxi concept is coming from Silicon Valley and not the traditional aerospace players (who, it must be said, are started to get onboard - especially Airbus). This creates pressure to develop a product that looks and feels revolutionary, to raise the billions required for aircraft development.  Public acceptance of these systems will also be an issue, and public opinion in many large cities is pretty anti-helicopter.

From a performance standpoint the big difference between most multirotor concepts and current helicopters is that lots of multirotor concepts are battery-powered (for noise, local emission, and infrastructure flexibility reasons). Batteries are much worse than avgas from a energy/weight standpoint.  A hybrid-electric multirotor would have broadly similar performance to a hybrid-electric helicopter, assuming they had the same disc loading.