I can build all the functionallity of a checkbox and a radio button with a select/dropdown, which results in a more lined up interface and I get around with lesser different types of GUI-elements.
So why should these elements be used anyway.
To replicate checkbox functionality (
choose 0 or x out of n), you'd need to employ a multiselect element. There's a lot of research out there indicating that most users are blissfully unaware of how to select multiple items in a multiselect element.
Which one to use, then? That really depends on your scenario.
Scenario 1: "Please select your country from this list of several hundred"
Scenario 2: "Do you like ice cream?"
tl;dr: Whether to use Radio Buttons or Dropdowns really depends on your use case. Every time you use a multiselect, a cute kitten dies.
For small numbers of options checkboxes and radios:
At first, checkboxes and radiobuttons make choices visible and don't require any additional actions to display the set of choices.
Second, their form displays without any text explanations the possibility to make one-from-many or many-from-many choice — and 'cause checkboxes and radiobuttons are recognisable, basic patterns, these elements have great affordance. In dropdowns and selects it will require additional cognitive efforts to understand the possibility of multiple choice, or any additional labels.
At third, dropdowns and choices can't display single checkbox choice correctly — usually it will require two choices ("yes"/"no") and a lot of actions instead of simple click.
And fourth, selects can be perceived as textareas, what makes additional confusion to users.
So, it is not so simple, and lined-up interface made in visual form made of selects and dropdowns will not show the difference in essence and will be much more harder to use.