In constraint systems for ZK proofs, why are multiplications counted but are additions not? For example, If I wanted to prove that:

$$x^2 + x^3 = 45$$

This cost of this would be calculated by counting the number of multiplications that need to be done, and not the addition of $x$ squared and $x$ cubed.
Your question seems to assume this is true for any constraint proof system. I'm quite confident that this property is on a case-by-case basis. Bulletproofs, for example, have this property: proof size is $\log(n)$, with $n$ the amount of multipliers in the arithmetic circuit. In the rest of my answer, I'll talk about how a bulletproof achieves this.

I'll give a very condensed sketch of how bulletproofs achieve this property. For a more in-depth explanation, I suggest to read the above paper, or the (in my opinion) simpler notes of the good folks at dalek-cryptography.

Bulletproofs writes their constraint system in a single vector equation like follows:

\begin{aligned}
\mathbf{W}_L \cdot \mathbf{a}_L +
\mathbf{W}_R \cdot \mathbf{a}_R +
\mathbf{W}_O \cdot \mathbf{a}_O =
\mathbf{W}_V \cdot \mathbf{v} +
\mathbf{c}
\end{aligned}

Here, the $\mathbf{W}_{\{L,R,O,V\}}$ are weight matrices: they map secrets from their right hand side to constraints. $\mathbf{v}$ are the input secrets. Note that this equation contains the linear constraints, not the multipliers. In what follows, they combine it with the multipliers ($\mathbf{a}_L\circ \mathbf{a}_R=\mathbf{a}_O$), and then this equation is molded into a form $t(x)=\langle\mathbf{l}(x),\mathbf{r}(x)\rangle$, in which $x$ is a challenge of the verifier. Here's the first clue: $\mathbf{l}(x)$ and $\mathbf{r}(x)$ are of length $n$, the amount of multiplication gates.

The proof process links $t(x)$ and the two vectors $\mathbf{l}(x)$ and $\mathbf{r}(x)$ to the secret input commitments (in constant size), and then goes on to prove that their inner product relation holds: the inner product proof. This was introduced by Bootle et al., in Efficient Zero-Knowledge Arguments for Arithmetic Circuits in the Discrete Log Setting, and improved upon in the bulletproofs paper. This is your second clue: this inner product argument that proves something of the form $t=\langle \mathbf{l}, \mathbf{r}\rangle$ needs $\log(|l|)=\log(|r|)$ communication.