It's not practical to manufacture plain nylon strings that are so thick that they could be tuned to the pitches of the three lower strings. Such strings would have to be so thick that they couldn't fit through the bridge holes, would not sit properly over the saddle, couldn't fit over the nut, and couldn't fit into the posts of the tuners. Furthermore pure nylon strings that thick would not hold a pitch properly; they couldn't be made to stay in tune.
This is the very reason that wound strings had to be invented -- to provide low pitches on a short-scale instrument. Wound strings were invented around the year 1660, so they've been around for about three hundred and fifty years. Up until around 1950, these strings were made of sheep gut, not nylon. But the principle is the same.
Before 1660, before they invented wound strings, if you wanted low bass strings, you had to have an instrument of much longer scale length.
Renaissance theorbo, with unwound bass strings of sheep gut.
This is a real musical instrument and some people still play them today.