Do you think most IT Managers get stagnant in same company for years? why?

It depends where does one work.
For dynamic and service oriented/innovative companies like IBM, Microsoft, Google, Oracle etc.
etc.
things are different as compared the companies which uses their products.
For example I'm working in Oil and Gas sector for the last7 years and am using the same software.
And believe me, I'll be using the same for many years to come.
If there is any change, it might be Windows Servers and Windows7 or8.
Infrastructure remain the same, challenges remain the same and people, almost remain the same.
So, my growth as IT professional comes to a halt.
If I have to learn new things, I must move to some different environment.
And HR professional suggest to think for change at least after5 years.
Before5 years, I don't like it.
So if anyone who has been on the same place for5 years and above, it is good idea to explore new vistas of learning.

It does happen in some environments where development of the company itself is stagnant.
But its the role of the IT manager to update and convince the management to evolve with the technology.

I think that you can't be efficient as an IT Manager if you don't know the whole IT infrastructure after several years.
I mean companies evolve, so does their IT systems.
Following a company and its evolution helps you understand its vision and how to apply it in IT.
And more importantly, after years, you get to understand the way employees work (management or not) and what they need.
I joined the company I'm working with around a year ago.
And I'm still learning/discovering new things about the corporate network (or about the IT infrastructure in general) simply because you can't know all the details and specifications from day 1.
So I won't say that you get stagnant.
I'd say instead that you grow and evolve with the company.

I understand that only those IT Managers get stagnant who make the IT Management as a routine job.
I understand the IT Managers should keep themselves up to date.
Encourage Innovative and new idea.
Keep adopting new methods, ways of working, processes or update existing once.
The Managers should become the Part of the Business Processes and Strategy,