They really aren't, though, especially not in lane, where they have very little synergy. In lane, Xayah likes to poke with her passive, Q, and E, as going aggressive generally doesn't work unless you're a god of feather placement.
Rakan, on the other hand, is kinda like Leona, but with more AoE lockdown and less single target. He can't really poke in lane due to his extremely short range (literally the shortest of any ranged champ), so he's better at going in on the enemy, which is something Xayah sucks at. Of course, he can also peel well, but Xayah doesn't need help with that, as she's more than capable of self-peel. The only thing Rakan can do that compliments Xayah's playstyle is counter-engage, which there are supports who are better for.
Outside of laning phase, their synergy gets slightly better, as Rakan can group up and lock down enemies to allow them to be hit by her ult and E, but
A) you'll never see anyone coordinated enough to do that in your SoloQ games, and
B) Other supports can still do that better.