Unlike hobby software, where you could reasonably write everything from scratch just for fun, or say in write it in 6502 ASM which hasn’t changed at all for 50 years and won’t change in 50 more, in modern business software, you use common frameworks, APIs, platforms. All of those mean the software you write will fail as those dependencies and platforms change and age out.

Business software has to be built on common building blocks because as a business you need to be able to hire people who can work on it. So things like Java and node and react and dozens of APIs exist which creates dependencies and requires ongoing maintenance as those components continue to change.