The legs on a Falcon have built in shock absorbers. Here is why. The energy with which you hit the ground is 0.5 m v^2. This has to match force times distance while the legs catch it. If it is perfectly rigid, the force is insane, even when rigid. But if the legs can flex, you can make this work out.
But if we've got a structure that catches you, then the rocket doesn't need to have those shock absorbers. The structure does. And if the longer the distance over which the shock is absorbed, the smoother the landing.
Now in Elon's plans, Starship needs to be able to land on Mars unassisted. So it actually needs legs with shock absorbers. But SuperHeavy only needs to take off from and land on Earth at pre-arranged locations. So he can make it more efficient by not building in any unnecessary shock absorbers.
Side note. This is why seatbelts matter. With a seatbelt your crash is spread over all of the time that the car is crashing. Without it your crash is spread over the time it takes your head to hit the car. When your head stops in 1/10 the distance you have to have 10x the force. It is not hard for this to be the difference between being shaken up and having your brains scrambled.
This is also true of Starship on Mars since it will be massing over 200 tons.
2. Right now, the legs need to be capable of absorbing the shock of the landing. With a dedicated platform, not only can the weight of the shock absorbers be added to the platform rather than the rocket but you can also potentially absorb over a dramatically greater distance and time (increasing the proportion of successful recoveries).