Why in the quantitative equation: $MV=PY$, $V$ and $Y$ can be taken as fixed? To  equation is
\begin{align}
MV=PY 
\end{align}
where $V=\frac{1}{k}$.
Why $V$ and $Y$ can be taken as fixed or constant?
Why can $V=\frac{1}{k}$ too?
Thanks in advance
There are multiple answers to this question.

In any model you can always make a thought experiment where you hold certain variables fixed.  So one answer, although not very satisfying one, is that you can view it as a thought experiment. For example, in physics distance traveled equals velocity times time or $D=tv$ and you can always make a thought experiment where you choose 1 or 2 variables that will be fixed and see how others behave.

If you are asking why it would make an economic/intuitive sense to take $Y$, $V$ fixed then:



Regarding $Y$: From economic theory we know that $Y$ in a long run depends on productive capacity of an economy that is independent of $M$, $P$ or $V$ (you can learn more about that in any 101 Macro/econ book such as Mankiw Macroeconomics or Mankiw Principles of Economics). So you can consider it to be exogenously given and holding it constant is just performing a thought experiment where you assume you have economy that is not growing so you can see how other variables behave.

Regarding $V$, in the past (before 2008), empirically speaking velocity of money was very stable (see the Fed graph below). As you can see between 60s and late 2000s it was always hovering around 1.9 so on empirical grounds many economists considered $V$ stable, although it is worth while pointing that recently it significantly declined. Nonetheless, given that it was historically very stable this might be just transitional, or it is possible there was a structural break and now velocity will remain approximately constant at some lower level. It is also possible that it was wrong to assume it is constant in the long run based on previous empirical observations.