Logical consequence of technical progress
I mean optics and routers have a limited lifespan. After 10-20 years maintenance costs will rise, which means you will want to replace them. Also if you want to get rid of your equipment earlier, there's a decent refurbishing market so you can recover a large fraction of your costs.
Plus equipment costs pale in comparison to the wages of the people maintaining them. A network engineer will easily cost you 100k$ a year if you count all costs directly associated with hiring a person.