Solution 1:

For example, $\pi$ and $1-\pi$ are transcendental, but $\pi+(1-\pi)=1$ is not.

Solution 2:

It is mainly because transcedental numbers behave so weird. For example, would you think that a transcedental number raised to an irrational is an integer? Well, it is possible:

$$(2^{\sqrt{2}})^{\sqrt{2}} = 2^{(\sqrt{2})^2}=4$$