There are two main reasons for this.
First, the Framers of the Constitution made it very hard to get amendments passed. An amendment must (in the most common procedure for amending the Constitution) be approved by two-thirds of each house of Congress and then ratified by three-fourths of the states. It is extremely difficult to get that kind of a supermajority for anything.
Second, the Constitution is meant to be a very basic document. It lays out the bare bones of American government but very rarely gets into the creation of specific policies. The one time the Constitution was changed to make a very specific policy was with Prohibition. For this reason, there is a general reluctance to use the amendment process to institute policies that ought (in the minds of most Americans) to be made through Congressional statutes.
The process by which the Constitution is amended is very difficult, time consuming, and overall tedious. This is to prevent random amendments from being applied, either giving the government or the people more power than is thought to be necessary.
If the government had the ability to add or take away from the constitution easily, then in theory a President could easily becaome a more powerful ruler.
If the people had the ability to add or take away from the consitution easily, then in theory they would be able to take over the government offices, and create chaos.
Either way, other amendments have not been deemed necessary enough to be made apart of the constitution.