They're kind of a rare spawn, but if you have a lot of paths it's harder as they can't spawn on a path. Also, they like flowers. The more flowers I had the more frequently they seemed to spawn. I tend to find them away from houses, and near cliffs, although the scorpion I found was right by re-tail so it may just be a coincidence.
I don't know if the flower thing is accurate. I'm re-landscaping and moved ALL of my flowers to the beach. I have some paths but my ground isn't covered in them. Running around the past few days I've probably seen about 8 lol. Maybe I just got lucky.
Keep going in and out of buildings. That's when they spawn: when you leave a building. So search, enter a building, leave, search again, enter a building, leave, rinse and repeat until you find one. It's a very rare spawn but if you're constantly attempting to spawn one it should pop up at some point.
And remember that when you're searching for a tarantula you shouldn't walk around with your net out. Otherwise, they'll sprint towards you and attack as soon as they see you.
flowers might be a problem, my tarantulas have escaped and disappeared into flowers numerous times. i've caught ~4 tarantulas and seen only 2 scorpions, caught one. the one scorpion suicided into the river. each time, i was not walking with my net out, and it was always quite late at night. my scorpion showed up at like 2AM?
spawn sites are pretty random, i've found tarantulas in various places. as everyone previously stated, it's just rare. good luck!