Why nrow function doesn t seem to work within summarise function

Question

See the code below:

library(dslabs)

data(gapminder)

differenceminder percent > percent

filter(year% in% c(2010, 2015) &!is.na(population))%%%%%%%%%%%%%%%%%%%%%%%%%%%%

percent > group by(year) percent

weight = nrow(population) in the summary

I keep getting the message, "Column weight is of unsupported type NULL," why?

Population is a vector inside the summarise() function, and nrow() accepts a vector as a parameter. I therefore wonder why it doesn't.

Note: I am not searching for workable substitutes for nrow() because I am aware of its alternatives in this situation. My query is philosophical and is focused on the word "Why."