Well, yes it doesn't look very high compared to others we can see from other brands.
However, here are a few elements to consider.
It was born in 1977 (starting to be a long time ago), with a nice micro-rotor, yet still at a 27.5 x 2.5 mm dimension. This size matter was very important for Patek as allowing smaller and more comfortable (lighter) watches. The bigger the parts, the longer energy has to travel, the more consumption (hence less accuracy or less PR).
A longer PR usually requires a longer mainspring (you can still work on the frequency), hence bigger spring barrel (like someone mentionned above).
Today, the evolutions of trends make that people prefer bigger caliber, especially as the watches get bigger ("to fill the caseback"). But that isn't coherent with an opinion that watchmaking would be about optimizing space, hence making as small as possible. The 1977's 240 is coherent with that philosophy.
It is very reliable and very accurate. A longer PR doesn't allways favor a better accuracy if not designed well enough.
The last manual 30-255 caliber has 2 barrels providing a 65h PR with 28,800 vph. It remains very thin but takes room. This new caliber is placed in the 6119 which is a 3 hander.
Having an automatic caliber doesn't make a shorter PR a problem if you wear a watch regularly. Of course, it can if you leave it aside for the week-end.
It is worth mentionning though that the PR is longer than stated as it is the PR that respects the best time keeping.
Thus, it depends on the wearer's habits.
For instance, the new 26-330 automatic caliber (replacing the 324 latest version) helped saving energy. When asking why they didn't increase the PR, they said the approximately 10 hours increase wouldn't really be helpful, for instance during a whole week-end (Friday to Monday). Hence they preferred improving accuracy instead.
You can have a look at the 26-330 article here (
https://www.hightime-reviews.com/pat...aliber-review/) if you want to see what research is put in efficiency and accuracy rather than elsewear. Watchmaking is first about precision, even if for some use we might not really need it (hence prefer PR).
The new 31-260 micro-rotor is a precision champion, yet a similar PR but with a 28,800 rate (more precise, especially when dealing with micro shocks).
A question of choice but at a certain cost, depending on the wearer's habits.
I hope this helps.