@robiin Diversification - e.g. if an issuer closes an ETF and I have to sell (taxes are not due on the entire portfolio), if an issuer gets into some kind of trouble, I temporarily cannot get the money but urgently need it, if the country of an issuer does strange things / enacts laws, ...
None of these are incredibly likely scenarios. But they could happen and I wouldn't be at a disadvantage (apart from changing my savings plan for 5 minutes). So why not further reduce a low risk?