The Mount Washington Hill Climb attracts hundreds of masochists every year to enjoy an hour or two of pain on a time trial up the highest peak in the northeast United States. Many of these riders modify their bikes for the race.
Most commonly over the years, riders who had a mountain bike as well as a road bike would temporarily graft parts of the mountain bike drive train onto the road bike. The crank would be stripped down to just the smallest ring. No need for the bigger rings on eight miles of continuous ascent. Ditch that hefty front derailleur and shifter, too. Likewise set aside one brake. Some riders even had a special wheel set.
To accommodate a wider cassette in the rear, the long-cage derailleur might make its way to the road frame for the day.
In the days of square, tapered bottom bracket axles, the cranks could swap over easily, as long as the granny ring cleared the frame. With a single ring in front, derailleur swing didn't matter. Perfect chain line wasn't a big deal either. After the start the rider would probably be in only a few gears near the inner end of the cassette anyway.
The conversion was cheap if the rider owned all the parts already. But then the process got Shimanoed.
With every added cog, spacing gets tighter. Splined bottom brackets for road didn't match splined BBs for mountain. Then came two-piece cranks with outboard bearings. Road has 10 speeds, mountain has 9. Brifters aren't really compatible with mountain derailleurs. Non-Shimano splined bottom brackets could have any of several spline patterns, because manufacturers did not embrace the ISIS format as the sole alternative to Shimano's proprietary antics. Conversion involves more parts and more shop time.
One conversion I've been working on will cost more than $400 for a one-time event. The owner of the bike needs to buy all the parts because he has no mountain bike to scrounge from. And he made it more complicated by insisting on two gears and a functioning derailleur in front.
It's all the same to me. I get paid by the hour. I'll figure out the details. He just needs to open the wallet.