Why are external hard drives better than USB flash drives for long term storage?

So you are getting a few terms combined which is causing confusion

Reliability wise

Tape drives are the most reliable

Enterprise SSDs are next

Consumer SSDs are after that

Hard drives are next

and flash drives are the least reliable

Flash drives (to make them cheaper) have no wear leveling and generally use cheaper flash which dies much quicker.

in perfect storage conditions a flash drive will lose data after 5-7 years un powered

hdds can generally last 7-10 before the disks start failing from disuse

SSDs can store data unpowered for up to 25 years (each degree C above room temp they lose 1 year of unpowered data retention, and each degree below C they gain 1 year of unpowered data retention up to 30 years max)

Tape drives can easily store data for 30-50 years unpowered (they are what most companies use to store data as they are very reliable)

Although USB flash drive is more durable, the external hard drive has a much longer lifespan if you take it carefully in that the hard drive is well and sophisticatedly designed. Therefore, if you mean to store your data for long term and don't need to use it frequently, the external hard drive is more suitable.

the internals of the thumb drives are much newer and the industry is pushing the materials to get maximum speed ... not maximum reliability

There are two main issues.

1. Flash memory has a limit on the number of times an individual memory cell can be written to before it fails. SSD drives have 'wear leveling' controllers to spread out the writes, cheap USB flash drives don't. Electron migration eventually kills the memory cells. 

2. Flash memory stores data as an electrical charge on the gate of a special type of MOSFET transistor. The charge will eventually bleed off if it is stored for a long period of time and the data will be lost.

Magnetic storage drives do not sufferer from memory cells wearing out like flash memory devices do and do not rely on electrical charge  to store data. The trade off is mechanical hard drives are more prone to mechanical damage. 

Yes, There are SSD. And nothing is reliable. Newer Computer comes with SSD instead of HDD. If data is important making its copy and storing on different devices is logical to the extend where price comes into play. So its really always Data Vs Price.