The synthetic equivalents have to be similar, because they're the same thing as the natural stones, and the natural stones are similar.

A ruby is corundum with chromium coloring it; a sapphire is corundum with (most typically) iron coloring it.

Iron isn't rare, so either it affects the process used to make rubies, or there's no real reason for the price gap.

Iron is not enough for sapphires, it must be combined with titanium.

I do not know how this is done, but while the doping with chromium for rubies is simple, chromium will just substitute aluminum in the crystal lattice and it will provide a color determined by its concentration, for sapphires there may be necessary a more complex thermal treatment, so that the iron and titanium will form the right type of coupled defects in the crystal, in order to have the desired color.

"No real" reason wouldn't shock me. Not a gemstone expert so I can't say, but if natural sapphires are more expensive/in higher demand than natural rubies (despite both being corundum) then it makes sense for that demand to map onto their synthetic counterparts. But with the synthetic stones, it's more obvious how arbitrary that demand/price difference is.

My impression was that natural rubies are generally more expensive than natural sapphires.

This is just speculation, but assuming you're talking about gem-grade rubies and sapphires, perhaps there's more industrial uses for similar rubies and the gem industry sees the benefit.