The controls were actually not as basic as you would think. Web controls still miss many features these had by default, such as Alt-underlined focus keys, searching through lists and dropdowns by typing the first letters, arrow key navigation in menu controls and treeviews, predictable behavior of Esc, tooltips, context sensitive help, consistent tab focus order and so on.

Their appearance is simple because of the graphical limitations of the time, but then again many modern UI frameworks are very much simplified too (they just use white instead of gray).