Probably because the Apple Watch is effectively a modern 10 year old smart phone made tiny with an ARM Cortex A series CPU running at nearly 2GHz while the Garmin is a ARM Cortex M series microcontroller doing ~200MHz.
That is the (modern) Apple way. They optimise for smooth experience first¹, other factors second.
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[1] Actually second, their priorities are money->UX->others - hence being uncooperative with any efforts to improve standard web apps despite the potential user benefits as they could compete with their appstore