Why different Shiny UI packages cannot be integrated within each other?

Question

Canned packages in Rshiny gives a stock appearance for any basic Rshiny apps. More advanced user probably opt HTML Templates. The canned packages I came across for Rshiny UI are Shiny Dashboard, Shiny Material Design and Flex dashboard. But, why it is not possible to call a Dashboard page and then have Shiny material design buttons, inputs or cards or vice versa ?

What prevents them from integrating and using across to customize apps ?

Something like the below code ( which does not run as wished for)

library(shinydashboard)
library(shiny)
library(shinymaterial)
ui <- dashboardPage(dashboardHeader(),
                    dashboardSidebar(material_row(
                      material_column(
                        width = 2,
                        material_card(
                          title = "",
                          depth = 4,
                          material_switch(
                            input_id = "trend_line",
                            label = "Trend Line",
                            off_label = "",
                            on_label = ""
                          ),
                          material_radio_button(
                            input_id = "plot_theme",
                            label = "Theme",
                            choices =
                              c(
                                "Default" = "default",
                                "Classic" = "classic",
                                "Light" = "light",
                                "Dark" = "dark"
                              )
                          )
                        )
                      )
                    )),
                    dashboardBody())

server <- function(input, output) {

}
shinyApp(ui, server)

Answer 1

It seems to be a current issue that you cannot mix and match, as some of the CSS stylesheet used in shinymaterial overlap with those of other packages such as some of the basic shiny inputs or htmlwidgets (see here).