Why do we use (pseudo) random permutations and not (pseudo) random functions for en- and decryption? Why do we use pseudo random permutations and not pseudo random functions for encryption an decryption?
We do use PRFs for encryption and decryption!  We use them as stream ciphers to generate one-time pads.

For example, with the ChaCha20 stream cipher, the encryption of the $n^{\mathit{th}}$ message $m$ in a conversation is

\begin{equation*}
  \operatorname{ChaCha20-Encrypt}_{\;k}(n, m) = m \oplus p,
\end{equation*}
where $\oplus$ is xor, and the pad $p$ is generated by
\begin{equation*}
  p = \operatorname{ChaCha20}_{\,k}(n \mathbin\Vert 0)
                \mathbin\Vert
                \operatorname{ChaCha20}_{\,k}(n \mathbin\Vert 1)
                \mathbin\Vert
                \dots.
\end{equation*}

$\operatorname{ChaCha20}_{\,k}$ is the ChaCha20 core PRF under the key $k$.

Likewise for the AES-CTR stream cipher, but we use the PRP $\operatorname{AES256}_k$ approximating a PRF instead of the PRF $\operatorname{ChaCha20}_{\,k}$ to generate each block of the one-time pad.  In all cases with this pattern, decryption conveniently turns out to be exactly the same procedure—which is a bonus for AES, because $\operatorname{AES256}_k$ is usually faster to compute than ${\operatorname{AES256}_k}^{-1}$, the latter of which is unnecessary to use in AES-CTR.

N.B.: If you're thinking about doing this in practice, please use it responsibly in an authenticated encryption scheme like crypto_secretbox_xsalsa20poly1305 or AES-GCM, not alone, unless you're very carefully designing an exotic protocol.