Why does restaurant wait staff take your menu when you've placed your order?

Nowadays, it's not so much an issue since many restaurants have digital menus but there are a few reasons.  
Most restaurants only have a limited number of menus.  They don't usually have one for every seat in the place as people will always be in different phases of dining process.  Worst case, they can give a table of 4 two menus instead of 4. So as soon as you order, they need to get the menu to the next table. 
As mentioned, most people want to get on to appetizers so no need for the menus taking up space.  
If you plan on having dessert, most places only have a couple so the server can rattle them off or they may have a small dessert only menu to bring later.  
You can certainly ask if you can hang onto the menu and I'm certain 99% of the time, they'll oblige. I do that at the Cheesecake Factory because it's such a huge menu.  

In order to make your table nice and uncluttered and also to free the menus up for other diners. I've never had a wait person deny my wanting to continue to study a menu. 

So they can give them to other diners. You can always ask to hold on to the menu to look it over more after you order.

To get it out of your way and so other patrons can use it.  If you’d like to keep looking then ask to keep it.   I’ve done so every now and again and it’s never presented a problem.  

Hang on to it then.

so you cannot change your mind after they have started making up your order

They have a finite number.  If it's a problem, just ask your server if you can hold onto the menu.  They typically don't mind.

So it is available for the next table........I am sure they would leave it if you asked, personally I am glad to see it go.

may be they think you don't  need the menu now because you already place order. So, they take the menu from you to provide that to the new comers.

It is mainly to clear the area and they ask if you would like anything else and if you say "no" they assume you are done with it.

Mostly because they think you have finished placing your order and it helps to clear away in preparation for your meals.  Just take longer to check out the menu before placing your order.  

why dont you get a brain instead of trolling .

lol good question. 

because they are greedy and in a hurry they want to do everything at once they don't care about your feelings