Bots don't need wards. They cheat. If you don't believe me, start a bot game and rush into a lane bush, then wait for them to move into lane. About half the time a bot will just "happen to" choose to ward your bush outta nowhere and they will immediately jump you. You will never see them randomly warding bushes with no one in them, or warding... pretty much anything else, in fact.
The ward is meaningless, the bots know where you are. They just programmed in the ward dropping to let players know that wards can be used that way and to make it seem a LITTLE less cheesy. Bots always know where you are, they're just programmed to ignore you sometimes if you're in bushes or the jungle. Making bots ward wouldn't really make them harder because bots are dumb. Sure, they know you're moving from mid to top but what do they do about it? Do they all mass top? Do they send one person to stall you while two of them take bot or Dragon? Do they ignore you entirely and all rush mid? Bots fail because the AI can't adapt it's strategy or playstyle like a person can. Giving them wards won't change that.
The only way they were able to make bots "harder" was the doom bots thing, which buffed them to the point of being outrageously OP, and they still lost a ton because they're too dumb to use that power effectively.