Saildrive-to-hull seals don't really wear. But they do age. They become brittle and start to develop cracks over time.
Volvo says to change them every 7 years because if they didn't say that, and a boat with 12 year old seals had them crack, leak, and sink the boat, you'd have:
Insurer: "So, owner, you never changed these seals?"
Owner: "No, we just followed the maintenance
schedule in the manual, which didn't say to change them."
Insurer: "So, Volvo, are these seals known to fail with age? Yes? And yet you led the owners into thinking they last forever?"
By explicitly saying "the seal only lasts 7 years, so change it then", Volvo is trying to protect themselves from being sued by insurers when the seals fail and sink the boat. Since it's now a routine maintenance
item, it's the owner's responsibility, and if it sinks the boat, the insurer will ask the owner "why didn't you follow the clearly-specified maintenance instructions for this thing?"