Well... you can. You only have to pay for this... a "small fee" of 150 USD per year!
You cannot modify PowerPoint's shortcuts, because years ago stupid Microsoft, hunger for money as usual, signed a stupid business agreement to make this feature available in form of paid plug-in only. Now you are a victim of everlasting eagerness of Microsoft and have two options: pay insanely high 150 bucks per year or live with default keyboard shortcuts only. This is at least 11 years old problem.
Yes, you are correct -- Microsoft enforces you to pay for changing keyboard shortcuts (plus 180 other useless features) the same amount (149 USD per) as you would actually pay for the entire Microsoft 365 Business Standard package (12.50 USD per month = 150 USD per year).
Can't find words to express what I am thinking about this idea and policy.