First, USB-C has been proven to allow for and practically reach 5Gbps or higher data transfer speeds in MacBooks.
Second, you may not see such transfer speeds for quite a while and there may be good reason for that.
A Samsung Galaxy or iPhone isn't a dedicated USB flash storage device. It (likely) doesn’t have low-power dedicated IC to expose the flash storage directly to a connected host.
If you connect your computer to an external SSD over a USB-C port, the computer will directly interface with the USB-to-SATA/NVMe/flash controller - whereas with a powered-on phone, it will only talk to its operating system (Android or iOS) with considerable associated processing overhead. Fast interfaces to the CPU or SoC don’t come for free in mobile devices. The chips used in such devices often don’t have spare PCIe lanes, additional USB3 controller chips or built-in USB3 connections, cause they cost money, space and power in such tiny devices. Last but not least, USB 3 has been known for potential (and sometimes quite observable in practice) interference with 2.4Ghz wireless connectivity. It’s surely not helped by the smaller physical separation between USB circuitry and antennas in smartphones (as opposed to tablets or notebooks).
Point being: there may be design and engineering choices why you aren’t getting very good USB data transfer performance on smartphones connected to computers. The engineers of such devices don’t set out to achieve the maximum theoretical transfer speeds on every single - wired or wireless - interface or protocol supported (though yes, sometime they do, for bragging rights …er marketing considerations). And that’s often for good reason, in such small and battery-powered devices.
PS: as much as I sometimes loathe Apple’s product differentiation, artificial gimping of products and “upgrade” pricing, USB data transfer speeds in iPhones aren’t really a big concern - unlike enforced MfI authentication on attached USB-C devices for money-grabbing purposes.