Excel and single-entry in general allows one to ignore questions about their financial structure, make decisions that lead to massive analytic information losses and bury a relatively good business in a deep grave of multiple “idk” levels of knowledge. They only become aware of it when it stops scaling (but competition does, for some reason) or a market goes down for a while (but competition doesn’t care, for some reason). Digging them out is a hard and tearful task.
I couldn't fit all of the transactions in a given month into Excel for my business. There are more than 1m rows. We are only an SME.
In many EU countries, if your turnover (total amount of money flowing through your business) is over a certain amount, or if you are incorporated (rather than sole proprietor) you have a legal requirement for double-entry accounting, and in some places a legal requirement to have an external firm do your accounting.