I'm not seasoned expert on DC/DC, but I've built a few buck, boost, flybacks, both with specialist chips and discrete designs from scratch. There are some which run at a lower frequency (LM2675 runs at 52kHz I believe). You've already stated the reasons for a higher switching frequency: smaller inductor/caps, as well as better transient response and more easily filtered for a clean DC output. But it comes at a cost of switching losess, and tighter layout specifications to minimise RFI/noise.
I'll admit, I haven't seen any SMPS chips using synchronous rectification that run at <100Khz. There could be many reasons, perhaps because modern MOSFET's can have sufficiently low gate charge to be switched at a higher frequency without significant power losess (especially with internal switches, as the IC manufacturer can control all aspects of the silicon). Also, what with many digital and analogue electronics sensitive to noise in the 10-100kHz, it may not be commercially viable to produce 'efficient sychronous DC/DC' chips, what with the expense of adding the sync switch and its control method, that might prove harder to filter than one jsut as easily produced that works at a higher frequency.
One more thought (I'm just thinking out loud here), the addition of a sychronous switch - or its control - in DC/DC controllers is a fairly recent thing, and may just coincide with better manufacturing that allows for higher switching frequencies at high efficiency. Not directly related, but both results of better technology that hit the market at a similar time.