Why didn't Voldemort hide his Horcruxes in impossible-to-reach places?

Probably the simplest explanation:

If they were impossible to reach, he couldn't put them there in the first place.

As it is, most of the Horcruxes were actually very well-hidden in a great variety of places. It was unlikely for them to be run across by accident, and less likely that anyone would know of more than one unless they'd gone digging into Voldemort's past - something that was probably very likely to get you killed, given how he's tried to distance himself from certain aspects of his past.

• Tom Riddle's Diary - Lucius Malfoy was among Voldemot's most trusted (and least insane) followers. He didn't know it was a Horcrux.
• Marvolo Gaunt's Ring - Under the floorboards in an abandoned house that everyone avoided, enchanted to make it more likely it would be avoided, and with the powerful curse on it so that whoever found it was unlikely to live much longer.
• Slytherin's Locket - That particular cave was only one of many along that cliff, and if I recall correctly, only known to Voldemort/Riddle and those he tormented during his younger years.
• Helga Hufflepuff's Cup - Hidden at Gringott's bank, there have only been 2 break-ins in its entire history: Quirrdemort and the main trio. Both after the Horcrux was hidden there. It is one of the most secure places in the world, one of the reasons the Stone of the first book was being stored there.
• Rowena Ravenclaw's Diadem - "The best place to hide a tree is in a forest", known on TVTropes as "Needle in a Stack of Needles". The Room of Hidden Things held a ton of random junk, making a small, single item very difficult to find, not to mention that Hogwarts school was considered among the most secure places in the world, hence the Stone being moved there in the first book.

Harry himself asked this question, and Dumbledore explained that it was Voldemort's titanic ego and flair for the dramatic that made him chose well-known items or locations.

The book implies that a person who was mentally all-together would have made that choice, but Voldemort, by the very fact that he'd make Horcruxes in the first place, wasn't all there, literally or figuratively.

I mean, digging a mile-deep hole in a random place in the ground, dropping the Horcrux in, and then filling the hole would be much more secure. Not nearly as dramatic, though, and certainly harder to regenerate from.

Erm...They WERE almost-impossible to reach places!

• Diary: Voldemort entrusted Lucius, one of his most trusted supporters. Lucius most likely hid it in Malfoy Manor, and I doubt the Malfoys would let just ANYONE inside their home.
• Gaunt Ring: It was hidden in a place where hardly anyone would have known to look. Plus it was under the floorboards. I don't know about you, but I find floorboards to be PRETTY impenetrable places to hide things. :3
• Locket: It was hind in a cave where hardly anyone visited, plus there were Inferius, and the poison in the cup was kinda impossible to drink without a partner to help you.
• Cup: Okay this is GRINGOTTS we're talking about here. Only ever been broken into once before Harry and co.
• Diadem: The Room of Requirement was scarcely known to anybody, and e Room of Hidden Things was massive and full of junk. There was probably more than one tiara in there.
• Nagini: Okay, not a hard to reach PLACE, but I'm sure Voldemort would've protected her well.

The way I always interpreted it, it was because the horcruxes had to be accessible for him to be resurrected (such as how Tom Riddle attempts to come back through a ritual involving the diary in the Chamber of Secrets).