You're confusing cause and effect here. In 2021, even crap 1% 10-year treasury notes cost $100 to purchase, not $80. It's only after interest rates went up in 2022 due to faster than expected Fed rate hikes that 1% treasury notes were no longer as valuable. An intuitive way to think about this is a seller of 1% treasury notes can only ask for $80 when 3% treasury note new issues are available for $100, to compensate the buyer of the 1% treasury note for the lower, less attractive interest rate. The interesting quirk is that it's only the mark-to-market price that is underwater i.e. worth $80. If SVB had the privilege to hold these assets to maturity (i.e. waiting 10 years), they could get their principal $100 back + $10 in interest.
If they did do what you said i.e. "spend $80 to buy $100's worth of treasury notes", then they wouldn't be in the conundrum they're in today. Because their cost basis in that case would be $80 and they could simply sell that treasury note for $80 and they wouldn't have a capital loss at all (in fact, they would be up since they earned coupon payments). It's only because:
1. 2021: they purchased low interest treasury notes before interest rates jumped
2. 2022: interest rates jumped faster than expected
This caused SVB incurring capital losses, which meant their assets to be worth less than their deposits.
If you are curious and want to visualize some of the changes on a graph, you can check out VFITX [1], which is a mutual fund that holds a mix of treasury bills, bonds and notes for an average of around 7-year duration.
-2021 Jan 1: $11.63/share
-2022 Jan 1: $11.12/share = 4.4% cap loss (before factoring in coupon payments)
-2023 Jan 1: $10.15/share = 12.7% cap loss (before factoring in coupon payments)
[1] https://www.marketwatch.com/investing/fund/vfitx