Because it is not easy.
And secondly to check if the earth is really not flat.
My bike is an old Marin trekking bike, I bought it as a second hand for about 250euro in 2009. The bike is about 14kg heavy and with all the equipment (tent, sleep back, spare inner tires) was about 31kg. I know exactly because with cartoon box I was just a little under the 32kg airport limit.
I used the best available tires from Schwalbe, a Marathon Plus, it works great.. however until you get on IS40. I had 3 -5 flats daily due to the truck tire debris lying on the shoulder. More about flats and the IS40 here >>IS40.