Visually, an LTO cartridge cannot have much above a half of the volume of a 3.5" HDD and it must be at least 3 or 4 times lighter.
Carrying a suitcase with twenty LTO cartridges is as easy as carrying any other typical suitcase, but carrying twenty HDDs would not be easy at all, due to the great volume and weight, and the volume in a real situation would be greatly increased by the need of adding some serious padding, in order to avoid catastrophic damage if the suitcase were accidentally dropped.
More importantly, the price per TB of tapes is much lower, and when you consider the ratio between the expected lifetimes, the price per TB and per year is much, much lower.
Of course, tape drives are extremely expensive, so you need to store at least a couple hundred TB to break even with the price of HDDs.