How often do miners recalculate the merkle root they're working on? I think that I understood most of the things about Bitcoin but there is still a mystery. 

We can see on https://blockchain.info/ that there are up to 4/5 new transactions every seconds. 
My question are : 
Do miners recalculate the Merkle root 4 to 5 times per second ? 
What happens to transactions made exactly as the same time a block is released?

I have seen that miners can choose the transactions they want to accept, but if it was the case I would be a miner, then accept maybe 2 or 3 transactions, find the nonce and get the 25 bitcoins reward. Why is that not possible?

I thank you a lot for your answers and I hope you will be able to help me. Thank you.
There are indeed many new transactions every second. The way miners deal with it is two-fold:


If an appropriate proof-of-work is found on any merkle root, simply publish that block and any transactions that did not make it into the block go into the next block (assuming sufficient fees)
Otherwise, calculate a new merkle root every so often (varies per miner, but let's say for the sake of argument that it's 5 seconds) and try to calculate the proof-of-work with the transactions included.


The reason why miners don't accept just 2 or 3 transactions is that it's just as much work to find the nonce for the proof-of-work whether there's 1 transaction or 1000 transactions. The probabilities remain the same. The nice thing about including more transactions as a miner is that you get more mining fees out of them.