Why we won't use i will instead of i shall?

u can, shall is more determine than will but both can be used in a sentence

According to rule, you have to use "shall" with "I & We". but when it is required to use "shall" with any subject you can use. eg:- वे लोग उपस्थित रहेंगे। They will be present. वे लोग अवश्य उपस्थित रहेंगे। They shall be present.

According to modern English, u can use both as will indicate more conformation in comparison to shall.