Because we as engineers create software for our customers, and if you don't understand who your customers are how can you create software that actually suits their needs?

Very rarely are we our own customers

I would argue that SREs are consistently our own customers in a way unique to SRE.

Ironic, as measurability came up in another comment thread I'm in.

I'd say from a technical perspective SREs are, but there's a potential (depends on product) gap between their technical goals and user goals.

e.g. What does "p95 latency is spiking" actually mean to the end user?