Why does grade inflation seem to be so severe in the USA? A comment on this question on undergraduate admissions notes that a good college might get 10x more applicants who all have the maximum possible grade point average, than the college can admit.  (The rest of my question is based on the assumption that is is actually true.  If it is not, my question might be based on an incorrect premise.)  Coming from The Netherlands, this sounds strange to me.  We have a standardised test at the end of secondary school, and few people score average grades higher than 8/10 (passing is 6/10), with people scoring 10.0/10 being practically nonexistent (I seem to recall reading about one such student in the newspaper many years ago).  The overall average is 6.4/10 for the school category granting admittance to university.  For universities I don't have hard numbers but I would be surprised if more than 1 in 30 students score 10.0/10 in undergraduate introductory calculus. Most students seem to score mostly around 6–7/10, which I believe would translate to E–D in the American system, but I'm not sure.  Grades 6–10 are passing grades, grades 1–5 are failing grades.  My question is for pre-university tests though, the ones that are used to test if people should be admitted to university.

Why is there so much grade inflation in the USA?  Is there no standardised testing for whatever tests are used to establish the grades used for undergraduate admissions?  This system would make it very hard for Dutch students to gain admission to American universities, because getting the maximum average grade is virtually unheard of.

See also: Do price and value of degrees correlate much?
There are a couple standardized tests used for undergraduate admissions that are generally taken near the end of high school. The ACT and the SAT are the most common ones.1 However, they are only part of the admissions package, and a relatively small part at that (varies by school, but my guess is they get a weight of around 25% or less at most schools).

There are many reasons not to base admissions solely on a single standardized test.2 One of the biggest is simply that standardized test scores often test studying and resources as much as they test intelligence and preparation. There's already a huge industry built around SAT-specific test preparation, and it's very difficult to argue that this extremely-targeted studying is doing anything beneficial for most student's academic preparation.

Also, keep in mind that the US is a much larger country than European countries, and there is a high amount of geographical mobility among the upper-/middle-class. Undergraduate enrollment in the US is around 18 million; that's larger than the entire population of The Netherlands. Because there is a huge prestige factor in going to a top school, almost all of the top students from that huge population will apply to the same 5-10 top colleges in the US. Even if the distribution of scores is the same as a European country, the much larger applicant pool means that top schools are inevitably going to be swamped with near-perfect applications.

1: The number of perfect SAT scores is actually quite low (I am less familiar with the ACT). In 2014, there were only 583 perfect scores out of 1.6 million test-takers. Though if you consider only the two more-important sections, there were 1,922 perfect CR+M scores. Even considering scores that are basically perfect (say, 1550+), less than half of 1% of test-takers reach that level. Source. However, that 0.5% translates into Harvard getting probably 5,000 applicants with 1550+ scores, which is already over 3x their regular incoming class.
2: The wikipedia article talks briefly about some of them, but there's a huge literature debating the advantages and disadvantages.