A tennis ranking is not an accumulation of career results. It is a rolling total over the previous twelve months, and that structure creates pressures that have nothing to do with current form.
Points expire on the anniversary of the result
When a player reaches a final, the points earned remain on the ranking for a year and then drop off entirely.
To hold position, he must replace them with a comparable result at the same time the following year, which is what players mean by defending points.
A player can win more matches this season than last and still fall in the rankings, because the comparison is against his own previous twelve months rather than against other players.
The calendar becomes uneven for each individual
Every player carries a personal schedule of when large blocks of points expire, determined by where he happened to do well.
Someone with a strong run in one part of the year faces a difficult stretch at that point every subsequent year until the result rolls through.
This is why two players with similar rankings can have completely different seasons ahead of them, and why one may be forced to enter tournaments the other can skip.
Mandatory events remove flexibility
The largest tournaments carry attendance requirements, and a missed event can count as a zero rather than simply not counting.
That reduces a player's ability to manage workload, since skipping a tournament for recovery carries a ranking cost as well as a competitive one.
Exemptions and protected rankings exist for injury, but they are limited and specified, and they rarely cover a full absence.
Ranking position determines opportunity
Seeding follows ranking, and a seed avoids the strongest opponents in early rounds, which makes deep runs more likely.
Direct entry to main draws also depends on ranking, so a player who falls below a threshold must play qualifying rounds before the tournament even begins.
These effects compound, because playing extra matches to reach the same stage costs energy that affects the result.
Why the system is defended despite the pressure
A rolling window keeps the ranking current, so it reflects what a player is doing now rather than what he did years ago.
Any longer window would leave declining players seeded above rising ones, which distorts draws and reduces the quality of late rounds.
The cost is that the ranking is a demanding treadmill, and much of what looks like a decline is a player failing to repeat a single exceptional fortnight.