It's illogical to say the PSS/SS titles are more difficult to win than the WC/OG when the same top best players, the main contenders, are competing.
We can only say the WC/OG - owing to its structure to include a wider range of players from various geographical areas for the sake of better representation and participation as befitting its title and purpose - makes it easier for the top contenders to get pass the early rounds where the weakest opponents are usually drawn to meet them. The final outcome is almost always the same handful of top players to emerge champions, with few exceptions.
Since 2007, the WC/OG is either captured by Lin Dan or Chen Long, with LCW a multiple finalist, the only exception was Paris 2010 won by Chen Jin against Taufik Hidayat.
A similar pattern emerges if you analyze the PSS/SS where more or less the handful of usual suspects, namely,LCW, Lin Dan, Chen Long, and a couple other near the top contenders, such as Kento Momota, J O Jorgensen, Viktor Axelsen. The rest can only hope to take the odd tournament when none of the Super Trio are participating.
The most sought-after trophies, the most-coveted titles and prizes are always the ones for which all the best of the best aim for and prepare the most for as they aspire to mount the winners podium and wear the crown. The logic is simple: first, what is the tournament for as the title suggests ? Second, equally important, is its rarity or frequency. Third, I'd say, are the stakes involved, prize, reward, increase in status, sponsorship deal, and suchlike.
For example, why is the WC worth more than any of the PSS/SS? Obviously, the World Champion status and its rarity, only once a year as compared with 12 PSS/SS of which 5 are premier and 7 normal.
However, beginning next year, BWF is unveiling a new tournament structure where the highest Level 1 events will comprise the World Championships, the Olympics, and one signature season finale with the highest prize money of USD1.5 million up for grabs. Expect every player to go all out for these three events of the highest status and, as a result, making them the toughest tournaments to win.