Because this wisdom is wrong. Investment is buying non-liquid capital, and to some extent, an irreversible decision; while saving is pretty much keeping liquid capital (like money) on hand.
So if you invest (make an irreversible decision), you are taking a risk that the decision is wrong. Therefore, other players on the market, who save and not invest, can learn from and react to your decision. In other words, keeping options open by saving (and not investing) gives you an upper hand over people who do invest.
Of course, if everybody saves and nobody invests, everybody loses as well. It's a prisoner's dilemma of sorts.
In practice this often manifests by large corporations keeping large amount of savings on hand, so they could "invest" in smaller companies that are deemed to be winners. It's generally a better strategy than to invest themselves everything back into some cutting edge research and risk these investments won't pan out.
The basic macroeconomic wisdom ignores this dynamic (and the fact that fundamental difference between investment and saving is that one is irreversible and the other is reversible), and so it concludes that all money that is not consumed (savings) must always be invested.
The Bank of England explaining it:
https://www.bankofengland.co.uk/working-paper/2018/banks-are...
[As a result NS&I cannot "fail" in the sense a commercial bank could, your deposits are guaranteed by the continued existence of the United Kingdom, and since it has its own currency this means if the bank fails, the country fails, the currency fails and the money was actually now worthless anyway.]
Interest rates being lower than inflation are a signal that pressures people to stop saving because the savings = investments equation has been broken by a huge margin.
The obvious question is, do these people hate money? Why aren't they just investing? One answer for that could be that all investments they want to make are illegal. Low carbon energy is an obvious investment. If you ban that and then ban the next thing, then what are all those pension funds (and other retirement related savings) supposed to invest their money into?