Why do we equate to zero when finding variables in a function? For example, given
$$y=2x+5$$
we let $y=0$ when finding $x$:
$$2x+5=0$$
$$2x=-5$$
$$x=-5/2=-2.5$$
Why do we set the value as zero when finding $x$?

You are misunderstanding the question you are being asked to solve. "$y=2x+5$" does not tell you what $x$ is. Maybe $x=5$ (in which case $y=15$), or $x=3$ (in which case $y=11$). So if I give you "$y=2x+5$" and then ask you to solve for $x$, the best you can do is $x={y-5\over 2}$.
However, let's say I ask you to find the zero of $y=2x+5$. This means that you're looking for the value of $x$ which gives $y=0$; so that's why we set $y=0$. Sometimes "root" is also used instead of "zero," here. Or, you could talk about solutions to $2x+5=0$. But this is very different from saying "the solution to $y=2x+5$ is $x=-{5\over 2}$," which is just nonsense.
