What are the benefits of purchasing a complete bike from a respectable brand? (Road cycling)
Warranty - buying one bike from one supplier means the whole thing is under warranty.  New bike frames may have anything from one year to lifetime warranty, but often have a rider about "must be assembled by an authorised and qualified bike shop"

Integration - all the parts will physically fit into the frame quite well.  Stuff like internally routed cabling simply works better when the parts are all specified on the one build.

Compatibility - Its not your problem to make sure that a front mech and a chainring set match up.  A complete bike will work from new as it is supposed to with no fiddling and fettling.

Set price - a complete bike has a set cost.  A build has an unknown end cost because you may not find the parts you need at a fair price.  On the other side, the parts you need might be in your possession already so have zero net cost.

Timeliness - you want to ride your new bike, not spend time assembling and troubleshooting it.  

Suggestion : Since you can afford a high-level bike like a canyon, and time is valuable, just buy it and ride it and spend the time on maintenance.