Because currently it has no real use.
I try to ELI5
The Bits define 2 things
1. How much size a number gets when “saved”
2. How many adresses the processor can know.
Lets start out with 1.
If you have a character in a role play game with a current amount of HP, you need to save this number somewhere. This is typically done in the RAM. Now if the max HP in the game would be 100, you could easily store this number in 1 Byte ( 2^8 Bits) which equals a maximum number of 255. But in a 32 bit system if the developer creates a 1 Byte Variable (placeholder for that number) this also takes the whole space of the 32 bit away. This means that you have to find a good amount how much Bits your Processor needs to have for your system, because bigger waste means you need bigger hardware (e.g. more RAM) which means more expensive system.
2.:
Every number of 1 gets saved somewhere, and every number gets an address. You can see the numbers equal to houses built in one large street and the address is the house number. Now lets say the postal office wants to send a letter to one of the houses. In a 8 bit system the postal office would have a small folder with adresses with only 255 pages. Everything that comes after this is basically invisible to the post office. Then they upgraded to 16 bit to be able to send letters to more adresses. This has been done with 32 bit and 64 bit again. If you calculate how much RAM you can have at maximum with 32 bit (2^32) you see that this is only 4GByte. This is nothing nowadays. Now calculate this again with 64 bit and you will see this will last long time.
When humanity finds a way to get incredibly much space for a cheap price, the next step will come amd we will jump to 128 Bits.
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