Why do we say there is a finite number of elements in the universe?

Because the number of subatomic particles is finite.

Explanation:

You probably know that in addition to electrons, protons, there are many other subatomic particles with strange names like quarks, neutrinos, muons, bosons, etc.

There are many ways in which these could theoretically be combined to make atoms of new elements.

However, the number of different subatomic particles is finite, so the number of possible combinations is also finite.

Even if scientists discover more new subatomic particles, the number of possible combinations will increase, but it will still be finite.

Thus, there is a finite number of elements in the universe.