All these sweepers are relatively weak, fragile and slow. Trying to set up with them AND keep rain up for the extra power/speed that you need is an uphill battle against anything but the most fragile teams.
First off, Rain-boosted water attacks are REALLY hard to take with anything but extremely bulky resists. Rain sweepers are anything but weak. I would also not say that it's very hard to keep rain up (unless your opponent has Y-zard, which Politoed hard counters anyway, or T-Tar who yes you should especially prepare for,) and most swift swimmers only need one round of seven turns of rain to lay waste to a reasonably weakened team.
252/0 Azumarill is 2HKOed by Kingdra's Specs Surf after rocks. 100/80 special bulk and resists, but it can't switch in. Kingdra itself has 75/95/95 bulk and two weaknesses (one of which is only ever used as STAB from things Kingdra can blow up with Draco Meteor/Dragon Pulse,) which is pretty good for an offensive mon. And with its ridiculous speed, it can invest in some HP.
The first post in this thread cited Keldeo as a problem for rain teams, but:
252+ Atk Life Orb Kabutops Waterfall vs. 0 HP / 0 Def Keldeo in Rain: 140-166 (43.3 - 51.3%) -- 65.2% chance to 2HKO after Stealth Rock
+2 252+ Atk Life Orb Kabutops Waterfall vs. 0 HP / 0 Def Keldeo in Rain: 281-330 (86.9 - 102.1%) -- 56.3% chance to OHKO after Stealth Rock
Can barely even switch into Kabutops, nor even check if Kabutops got a turn to boost. Kabtuops also has respectable 60/105 physical bulk (we won't talk about that SDef tho.) Also just a fun fact, Adamant Kabutops' Rain-boosted waterfalls hit slightly harder than Jolly Garchomp's Outrage.
And of course, no bulky waters are going to be able to take on Ludicolo.
And this is only mentioning Swift Swimmers, who are not the only mons rain teams use. They're certainly the scariest cause of how stupid fast they are, but they're better left as a win condition since they're the ones able to do the most damage in so few turns, and are the hardest to check. But rain also has:
-Crawdaunt: You do no switch into Crawdaunt under rain. You just can't. Banded Crawdaunt can 2HKO 252/252+ Mega Venusaur after rocks, which is absolutely insane. It has a chance to OHKO Azumarill after rocks, which is also really ridiculous. Pretty much only Chesnaught can hope to switch in but it has to be at absolutely full health to do so. You want to fuck stall? Use this. Or Gothitelle, but Gothitelle doesn't have Adaptability Aqua Jet which also gets boosted in rain. Slow and frail tho, but is not overly dependent on rain because it hits so hard anyway.
-Azumarill: Not so frail. Not so easy to wall in rain. Superpower nails Ferrothorn, Play Rough nails most bulky waters and grasses, so this just leaves Mega Venusaur and Amoonguss (and some really, really bulky water resists like Tangrowth.) Everything else is 2HKOed by CB set (Skarm takes 61.3 - 72.7%, for example.) As mentioned earlier in the thread, Belly Drum can wreck offensive teams. Also not so dependent on rain.
-Keldeo: Respectable bulk for an offensive mon and is also really fast. Has Secret Sword for Chansey of course, as well as Ferrothorn. This is, again, something that's not that dependent on rain, and can function without it.
-Electric types: Mega Ampharos is just a bit less bulky than your average wall, with a lot of useful resistances, and in rain it can freely spam Thunder, which hits like a ton of bricks off of its huge SpA plus it has Focus Blast for Ferrothorn. Mega Manectric isn't so bulky, but it's fast as balls and its Thunder hits like a smaller ton of bricks, but sadly its Fire moves (one of the reasons it's good in OU) are gimped in rain. Both of these have Volt Switch which is really good for regaining muhmentum.
-Steel Types: Fire weakness reduced/neutralized, blah blah. Ferrothorn is good to use as it counters most Azumarill and sets up hazards against Mega Venusaur.
Excadrill + Tyranitar works because they're both good even when not relying on the weather. Politoed + X doesn't have that kind of synergy.
Politoed is kind of ass I agree, but it's not that bad. Encore is trolly. And again, Rain has multiple things it can use that are not that reliant on having rain.
Excadrill is good, but this is mostly because it can run Scarf and Rapid Spin sets. Sand Rush sets with Quake/Slide/Head/Swords are not particularly good if Sandstorm is not up; don't kid yourself, Excadrill is as dependent on T-Tar as Swimmers are for rain setters, it's just that T-Tar is really good.