Encounter designers need to be able to rely on tanks having certain tools to interact with mechanics. If they, for example, took the 120s cd away from one tank for iDeNtItY or whatever then the job is no longer functional. The opposite is true as well: If you give a tank more tools than the others then those tools can never be needed or that tank is now the only viable option. For example there will never be a mechanic that relies on Cover+healer lb3 since only one tank can do it. Even small variations like the length of the invuln CD means that there can never be a fight where Holmgang's 240s is required or PLD just wouldn't be able to do it with it's 420s Hallowed Ground.

There's just not that much you can do with tanks.