Every service business begins the same way, and the problem is that the beginning works.

You do something well for one client. A second client wants the same thing. The obvious move, and on that day with that information the correct one, is to copy what you built for the first and edit the copy. It takes an afternoon instead of a week, the client is delighted, and you have doubled revenue at almost no marginal cost. It feels like the leverage everyone promises you.

Do it eight more times and ten near-identical versions of the same logic are running in ten places, diverging, and a fix to one reaches none of the other nine. The bug you patched for client three is still live for clients one, two, and four through ten, and you will find out which when one of them calls. The margin that looked like it was compounding has been converting into maintenance, and the fix now costs a migration performed on live systems belonging to people who are paying you.