Why is alloy considered as a mixture?

Hint: An alloy is a homogenous mixture of two or more metals or non-metals or nonmetals mixed in a molten state. It can either be a solid solution of the elements of a single phase or a mixture of two or more metallic phases. Solid alloys give a single solid phase microstructure, and are therefore used where their properties are superior to those of pure component elements. Some examples of alloys include - steel, solder, brass, amalgam, etc.

Complete answer:
Alloys are homogeneous mixtures of metals and cannot be separated into its constituents by physical methods. Alloy consists of metals mixed in a particular proportion. They exhibit the properties of the constituents. Alloys are mixtures because the elements that make it up are physically together, not chemically combined or joined together as in a compound. No new chemical properties are shown once the metals are mixed. So, an alloy retains the properties of the constituent elements. It is considered a mixture because it shows the property of its constituents.
 Hence, an alloy is considered as a mixture.

Additional Information:
Many alloys that contain silver in their name usually do not contain silver as one of their constituent elements. They are called 'silver' only because of their color. Examples of this are Tibetan silver and German silver.
Stainless steel, which is formed by mixing iron, small quantities of carbon and chromium is called 'stainless' because chromium gives it resistance from any kind of stain or iron rust. The chromium reacts with oxygen to form a thin layer of chromium oxide, which acts as the protective layer against any further corrosion.

Note:
The primary metal used to make an alloy is called the base, the matrix, or the solvent. The secondary constituents are known as solutes. The entire range of possible variations is called a system, (since the percentage of each constituent can be varied with any mixture).
 All of the forms of an alloy containing only two constituents (like iron and carbon) is called a binary system. All of the combinations possible with three alloys (such as alloys of iron, carbon and chromium) is called a ternary system, etc.