Why cannot mine more than ~21000000 bitcoin?

To answer this question let’s start introducing this concept:

HALVING

As the word says, it divides by 2…what does it divide?
Each time a block is mined, who found the right hash will receive a reward, at the beginning of the mining process, bitcoin blockchain’s reward was 50 BTC. The limit of 21000000 is not encoded anywhere, what is econded is process to halve the reward, each 210000 block mined the reward will be divided by 2.

By the time I’m writing this article the last block mined is the number 558822 and the reward has been 12.5 BTC.

Another important parameter to calculate the maximum number of possible bitcoin is the TIME.
An alghoritm guarantee that each block is going to be mined at an avarage of 10 minutes.

Last important information is the SATOSHI. 
Satoshi, beside being considered the bitcoin creator, is also the minimun indivisible amount of BTC and it correspond to 0.00000001.

~21000000 is the number of possible mined bitcoin.
As we said at the beginning of the article, bitcoin blockchain reward started with 50 BTC, so easy calculation 210000 x 25, then after the first halving other 210000 x 25….and so on.
We can summarize that sum and multiplicaiton with the following:

One thing missing…the sum does not continue until infinite, but as we said the mininum amount of bitcoint is the satoshi…so we can conclude that it is not exactly 21000000 but ~21000000.

Some infos before the end of the article.
Next halving should happen in May 2020 unless some wrong calculus and the price could raise up for obvius reason, mining a bitcoin will cost more!!!