That is not a column, those are row_names, Excel doesn't understand that concept that is why you get (1, 2, 3, ..n).
You could manually assign any column as row_names if you need to for some reason, see this example
library(dplyr)
df <- iris %>% group_by(Species) %>% summarise_all(max) %>% as.data.frame()
df
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1 setosa 5.8 4.4 1.9 0.6
#> 2 versicolor 7.0 3.4 5.1 1.8
#> 3 virginica 7.9 3.8 6.9 2.5
rownames(df) <- df$Species
df
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> setosa setosa 5.8 4.4 1.9 0.6
#> versicolor versicolor 7.0 3.4 5.1 1.8
#> virginica virginica 7.9 3.8 6.9 2.5
Created on 2019-05-01 by the reprex package (v0.2.1)