Because story points is a estimation statistic and it is supposed to be used for estimation i.e before the start of sprint. If you change story points in middle of sprint then it is classified as error in estimation.
All the reports will only consider the story points you specified at the start of sprint. Velocity chart will only burn down that original story point value. Any change to it during sprint will not be recognised.
If you recognise that a story was underestimated then instead of updating the story points i recommend you update the remaining estimate and that way at least your burndown will be representative of the new change.
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