There was a time, 50 years ago, when this would not matter, much. Banks were at the time relatively small businesses. They did not appear high in the FTSE.
50 years ago is – *checks calendar* – 1973
FTSE100 was launched in Jan 1984.
The precursor, the FT 30, deliberately and specifically excluded financial sector corporates like banks. Tho’ that did change and NatWest, Lloyds, RBS all joined it.
So, currently FTSE100 has 5 banks in it – Lloyds, NatWest, Standard Chartered, HSBC and Barclays. Or 5% of it. FT30 had, by the end of it, 3 banks, or 10% of it.
Banks are currently less of the stock market index than they were.