Are there any grandmasters who always play until checkmate? Are there any high-level players who are either known for playing games out until the bitter end or have done so in famous circumstances? Is this considered poor etiquette (I'd imagine it would be rude to force a master to play out a simple king+queen vs. king checkmate)?
To become Grandmaster, you must win many tournament games. They take many hours, sometimes multiple games per day, or many days in a row. Tournaments are exhausting. If you are tired then you make mistakes, and making mistakes means you don't play at the required level to become Grandmaster.

Players who always waste a lot of time by playing on until checkmate instead of resting aren't very likely to become Grandmaster.