Why it happens
Why the word limit is not a suggestion

The short answer
Because the failure is mechanical, not editorial. Some portals truncate at the limit mid-sentence and submit whatever fits; some committees stop reading at the stated length. Neither is recoverable after the deadline, and both happen before anyone has judged the quality of your argument.
The two ways over-length submissions fail
Silent truncation. A portal with a character cap accepts a paste and cuts it, and the applicant never sees the submitted version. The close, which is where your plan lives, is what gets lost.
Reader fatigue. Where the limit is a stated expectation rather than an enforced field, a committee reading a hundred files gives the over-long one less attention, not more.
Characters and words are not the same limit
UCAS counts 4,000 characters including spaces, which is roughly 600 words rather than the 800 people assume. Always paste into the actual field and check before the deadline.
How to cut without losing the argument
Cut description before evidence. Then qualifiers. Then anything a reader could get from your CV. What survives should be claims and the things that prove them, in the proportions the structure calls for.
- Sentences that establish context more than once
- Adjectives with no evidence under them
- Restatements of your CV
- Any sentence that would be true of another applicant
Related questions
What happens if I go slightly over the word limit?
It depends on whether the field enforces it. Where a portal caps characters, anything beyond the cap is silently cut, usually taking your closing paragraph with it. Where the limit is stated but not enforced, going over rarely disqualifies you but consistently costs attention.