Because, according to the definition of rational number:

A rational number is the ratio of an integer A to a positive integer B, for example, 3/8, and the general rule is A/B. A rational number is a set of integers and fractions, and an integer can also be regarded as a fraction with a denominator of 1. The fractional part of a rational number is a finite or infinite cyclic number.

π=3. 14 15926 ... is an infinite acyclic decimal, which is not within the range of rational numbers.

Extended data:

Infinitely cyclic decimals are also called irrational numbers. It doesn't write the ratio of two integers. If written in decimal form, there are infinitely many digits after the decimal point, which will not cycle. In a positional number system (for example, in a decimal number or any other natural basis), the representation of an irrational number will not be terminated or repeated, that is, it does not contain a subsequence of numbers.

Common irrational numbers are: the ratio of circumference to diameter, Euler number e, golden ratio φ and so on.

References:

Irrational Numbers-Baidu Encyclopedia