Completely random gates don't make sense from a lore wise perspective.
http://www.imperial-library.info/obbooks/liminal_bridges.shtml details (it's a bit of a thesaurus job) how you can only open portals to Oblivion from the inside but that these aren't very stable. Though not mentioned in the book, this is obviously because of the Dragon Fires. Now that those are gone, the portals can remain open longer.
To open a portal you need a Sigil stone. Presumably the same Sigil stone you can find on top of an Oblivion tower. Because there is no realm or tower without a Sigil Stone, this implies that there is no such thing as a random gate. Every one of them has been opened manually.
So I'd chalk it up to gameplay that there were so many gates. They're basically just another type of dungeon to go through.
Umm, that does look a bit awkward:
Let the mechanic prepare a chamber, sealed against all daylight and disturbances of the outer air, roofed and walled with white stone and floored with black tiles. All surfaces of this chamber must be ritually purified with a solution of void salts in ether solvent.
A foursquare table shall be placed in the center of the room, with a dish to receive the morpholith. Four censers shall be prepared with incense compounded from gorvix and harrada. On the equinox, the mechanic shall then place the morpolith in the dish and intone the rites of the Book of Law, beginning at dawn and continuing without cease until the sunset of the same day.
Clearly has to be done on Nirn if this is how it was done .
To open a gate to Oblivion, the mechanic must communicate directly, by spell or enchantment, with the Daedra Lord who inscribed the sigil stone in question. The Daedra Lord and the mechanic jointly invoke the conjurational charter [2], and the mechanic activates the charged sigil stone, which is immediately transported through the liminal barrier to the spot where its sigil was inscribed, thus opening a temporary portal between Mundus and Oblivion. This portal may only remain open for a brief period of time, depending on the strength of the liminal barrier at the chosen spots, several minutes being the longest ever reported, so the usefulness of such a gate is quite limited.
These seem to be the bits - morpholith being rock or stone of change or potential I suppose. And it seems that Mankar had to travel to where Dagoth was to get each of his stones inscribed.
However I favour the chaos theory re the choices of locations. See, Dagon had this very popular lieutenant who annoyed him - so he called the lieutenant and all of that lieutenant's supporters to him and told him he was going to honor him greatly by allowing him to find the locations of the places where random gates were to be opened. And the multitude of supporters were very pleased with this.
Before him on a vast table covered with a map of Tamriel Dagon had piled 99 grains of rock. Dagon bangs his fist down on the table and the grains of rock scatter and then asks his lieutenant: "How long do you think it will take you to locate those grains of rock on the map?"
The lieutenant takes a look and says: "about two hands sire".
"Very well", Dagon says, "I trust your judgement, that's why i chose you for the task, and just to make this interesting I am going to reward you handsomely if you complete the job in under two hands, but I will execute one of those gathered here and just to be fair subject you to numerous unspeakably violent tortures for every hand that you take over that time".
"Thank you sire", says the lieutenant, "but how will we know when I have located them all"?
"Oh, that's easy," smirks Dagon, "just make a mark on the map for each one and bring me all one hundred of them".