Just for brevity, given we’ve discussed this elsewhere before: for many people an ISA is just a want and, especially, when it’s constantly in the news to use the ISA allowance - be it cash or otherwise - it’s simply prudent learned behaviour to prefer an ISA. Convenience has value and inattention is real in the economy: consumers simply don’t want to have to think about frictions such as the CGT etc. An ISA is a good option simply for peace of mind alone.
Furthermore, ISA allowances are split across the number of ISAs one has which is another factor.
Whilst unlikely, it isn’t out of the realm of possibility. Take OCADO that jumped 70% this week. There are many examples of stocks doubling… so whilst ill advised to go all in on a few stocks, when doing so market-beating returns are possible.