Goes back further than the 90s. I would say it started in the 70s when the entire idea of lifetime employment at one company went out the window.
I think that was a result of the start of the integrated investment firms, derivatives, and mergers :
https://www.wallstreetprep.com/knowledge/the-history-of-investment-banking/
1970-1980
In light of the repeal of negotiated rates in 1975, trading commissions collapsed and trading profitability declined. Research-focused boutiques were squeezed out and the trend of an integrated investment bank, providing sales, trading, research, and investment banking under one roof began to take root. In the late 70’s and early 80’s saw the rise of a number of financial products such as derivatives, high yield an structured products, which provided lucrative returns for investment banks. Also in the late 1970s, the facilitation of corporate mergers was being hailed as the last gold mine by investment bankers who assumed that Glass-Steagall would some day collapse and lead to a securities business overrun by commercial banks. Eventually, Glass-Steagall did crumble, but not until 1999. And the results weren’t nearly as disastrous as once speculated.
See also here: https://www.investopedia.com/articles/stocks/09/stocks-1950s-1970s.asp
And in the 1980s the leveraged buyout market exploded giving us KKR, Michael Milken, Carl Icahn etc...