Why is there 1024 MB in 1 GB?

I looked on google and found out that there is 1024 MegaBytes (MB) in a GigaByte (GB). I thought it was quite an irregular number, so I was confused. Why dont they use a number like 1000?

P.S. My mum kept rambling on about the "shape" of a GB, thinking about a couple of cubes stuck together.
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