Why Dispose is not called even with using-statement?

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As per MS Docs: try-finally (C# Reference)

Within a handled exception, the associated finally block is guaranteed
to be run. However, if the exception is unhandled, execution of the
finally block is dependent on how the exception unwind operation is
triggered. That, in turn, is dependent on how your computer is set up.

As you are not catching the DivideByZero exception and let it be unhandled, on your machine and setup it must be bringing down the application before any other line of code is run and therefore not running the finally block.

As @Evk has pointed out in below comment, if I run it without attaching debugger, it unwinds the exceptions correctly and executes the finally block. Learn something new everyday.

As per Eric Lippert’s answer to Finally Block Not Running?

Think about how awful that situation is: something unexpected has
happened that no one ever wrote code to handle. Is the right thing to
do in that situation to run even more code, that was probably also not
built to handle this situation? Possibly not. Often the right thing to
do here is to not attempt to run the finally blocks because doing so
will make a bad situation even worse. You already know the process is
going down; put it out of its misery immediately.

In a scenario where an unhandled exception is going to take down the
process, anything can happen. It is implementation-defined what
happens in this case: whether the error is reported to Windows error
reporting, whether a debugger starts up, and so on. The CLR is
perfectly within its rights to attempt to run finally blocks, and is
also perfectly within its rights to fail fast. In this scenario all
bets are off; different implementations can choose to do different
things.

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