Any reason for that?
The reason is that.. it's not, actually!
Like, individual websites might do their own weird takes and have their own design systems which are mistaken for 'material design', but I don't think you can fault Google for making text boxes too similar to buttons.
Text fields https://m3.material.io/components/text-fields/overview - underlined or outlined, have their title text
Buttons https://m3.material.io/components/all-buttons - fully colored or outlined, may have a shadow, more rounded, differently-styled
One of the tenets of material design seems to be that a rectangle should not reveal its true nature until you click on it. It might be a button, a text box, or just a rectangle!