R为什么Tibble的列表显示为Tibble的列表，而Tibble的向量显示为Tibble的单独列？

library(tidyverse)

t1 <- tibble(num = 1:7, ll = letters[1:7])

t2 <- tibble(num = 8:14, ll = letters[8:14])

t3 <- tibble(num = 15:21, ll = letters[15:21])

vecs <- c(t1, t2, t3)

lis <- list(t1, t2, t3)

vecs %>% str
#> List of 6
#>  $ num: int [1:7] 1 2 3 4 5 6 7
#>  $ ll : chr [1:7] "a" "b" "c" "d" ...
#>  $ num: int [1:7] 8 9 10 11 12 13 14
#>  $ ll : chr [1:7] "h" "i" "j" "k" ...
#>  $ num: int [1:7] 15 16 17 18 19 20 21
#>  $ ll : chr [1:7] "o" "p" "q" "r" ...

lis %>% str
#> List of 3
#>  $ : tibble [7 × 2] (S3: tbl_df/tbl/data.frame)
#>   ..$ num: int [1:7] 1 2 3 4 5 6 7
#>   ..$ ll : chr [1:7] "a" "b" "c" "d" ...
#>  $ : tibble [7 × 2] (S3: tbl_df/tbl/data.frame)
#>   ..$ num: int [1:7] 8 9 10 11 12 13 14
#>   ..$ ll : chr [1:7] "h" "i" "j" "k" ...
#>  $ : tibble [7 × 2] (S3: tbl_df/tbl/data.frame)
#>   ..$ num: int [1:7] 15 16 17 18 19 20 21
#>   ..$ ll : chr [1:7] "o" "p" "q" "r" ...

创建于2023-03-22带有reprex v2.0.2
我希望列表和向量被同样对待。