In reference to the gregorian calendar, Why science people did not add an extra day to the year 2000?

The rules around leap years are quite complicated, and and the rule is NOT to just add a day every 4 years. That is the Julian calendar, and is not in use.

If you just added a day every 4 years, that would mean a year is 365.25 days exactly, and it is not.

wikipedia:

The Gregorian calendar modifies the Julian calendar's regular cycle of leap years, years exactly divisible by four, including all centurial years, as follows:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

This gives an average year length of exactly 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds.

PS, The year 2000 WAS a leap year, and Feb had 29 days.

.

Because the time taken for the earth to spin on it's axis (a day) doesn't divide exactly in to the time it takes for the earth to go once around the sun (a year). Which is why we add a day every 4 years to try and get a closer approximation and they fit a bit better; but this too starts to wander off when we are looking at centuries and millennia, so deciding when to add a day or not is just a way of trying to get them in line. The earth too speeds up and slows down as we found out when we created atomic clocks. I heard recently with all the dams made in the northern hemisphere the extra water trapped has made the earth speed up a little. In terms of a humans life we wouldn't notice it anyway.

Because the year is only 365.2424 days, not 365.2425.