One reason would be that they are less common in the UK – we don’t have a real equivalent of the local and regional Olympiad in academic subjects.
The Oxford and Cambridge admissions tutors we’ve met openly admit they can be decisive in applications. They also show commitment to a subject and ‘going beyond the curriculum’ along with all the usual generic skills such as performing under pressure and (in some cases) team work.
There is also nothing wrong – and a lot right – in admitting that you are a competitive person and thoroughly enjoy these events.
It depends on what subject you are applying for.
With the academic subjects, especially maths and the sciences, the Olympiads or similar national contests,are the one to go for, especially if you are applying to Oxbridge or other elite universities. Success in the English national obviously helps to demonstrate your ability in this language.
On-line completions, such as Codeforces can be strengthen Computer Science applications.
Other academic competitions are also useful. English Public speaking and debating competitions can strengthen any applicant’s application.
Drama competitions have made a critical difference to students applying for both drama(theatre) studies and film studies.