Why do airplanes and on their back wheels first and then on their front wheels?

Many aircraft land on their front wheels. Admittedly these tend to be older aircraft and certainly smaller aircraft than many modern aircraft. They are known as taildraggers. They land on the front wheels and then let the tail drop after they lose speed. These have several inherent difficulties. Firstly they are much easier to tip forward impacting the propellor on the ground during landing or take off, secondly, visibility is usually very very bad for the pilots while taxiing (They have to weave to keep sight of where they are going) and thirdly they are less efficient during the low speed part of the take off run.

But there are some other very good reasons for most modern aircraft to have a conventional tricycle undercarriage arrangement with initial touchdown on the rear wheels. Mainly due to angle of attack.

During take off, while all wheels are on the ground, the airflow over the wing means the aerodynamic drag (and lift) is minimised until you raise the nose at take off speed. This means that the aircraft can accelerate as quickly as possible while lift is not required minimising the necessary runway take off length.

During flight the aircraft can be designed so that the fuselage is nearly level at cruise speed making moving around in the aircraft more natural and less of an effort.
During a hard landing as you touch down the nose drops reducing the lift making bounces less pronounced.

There is no risk of tipping up by applying too much brake during the landing run.

These tricycle undercarriaged aircraft do not tolerate landing nose wheel first. They are not designed to take the impact forces of all of the aircrafts weight and momentum on the nose. So the nose wheel will very often collapse when the aircraft lands first on the nose. Small light aircraft may get away with it initially but if the nose wheel touches the runway first then it bounces the nose up into the air while the main wheels are still descending, this change of attitude gives a huge increase in lift (and drag) and the aircraft bounces back into the air, where it rapidly reaches a stall, the nose drops and the aircraft accelerates back to the runway where the nosewheel hits again. If the nose wheel survives then the plane gets bounced back into the air again to repeat the cycle, otherwise the nosegear collapses and the propellor hits the ground bringing the aircraft to a rapid stop.

See an example of this kangaroo type landing.