The RMS Olympic started service in 1911 and ended service 24 years later in 1935. The [RMS Mauretania (1906)](https://en.wikipedia.org/wiki/RMS_Mauretania_(1906)) ended service in 1934, after 28 years.
These ships were a huge investment, and people really liked them. The demand was still there, as airplanes still hadn't taken over transatlantic service. Cunard White Star launched a second RMS Mauretania in 1938. It seems weird to do this instead of keeping the old one and just updating it as necessary.
Was there some reason why these ships simply couldn't last longer?
The Queen Elizabeth 2 started service in 1969 and ended service in 2008. That's a more reasonable 39 years.
The wiki article you linked seems to answer your question to a certain degree:
Olympic returned to civilian service after the war and served successfully as an ocean liner throughout the 1920s and into the first half of the 1930s, although increased competition, and the slump in trade during the Great Depression after 1930, made her operation increasingly unprofitable.
So my follow up to your post is to ask for clarification about this sentence. "Increased competition" does indicate there was still demand. However what this particular ship just unlucky, more expensive to run/maintain or simply became obsolete super quick? Also what effect did WWI have on the trade - were there just a lot of surplus ships around which also were able to be converted (back) to civilian purpose?