Getting a very good lockdown mode requires both owning the entire stack (Apps + OS + Silicon) and being willing to sacrifice repairability (swapping chips/cameras/displays/touch controllers is a good way to help hack into a phone), and willingness to spend a lot of money on something that few people would actually pay for. Apple is the only company that's even positioned to take on this challenge.

AndroidOS has to work with a bunch of core functionality chips that Google/Samsung don't make. Having a bunch of different code paths/interfaces for a bunch of different SoC's, cellular modems, touch controllers, and cameras is not a winning recipe for security. Both Google and Samsung also use their own SoC's (Google Tensor G5, Samsung Exynos) but Samsung also uses a lot of Qualcomm Snapdragons ... and if you're using someone else's SoC there's no chance in hell of coming up with a proper "Lockdown Mode". Samsung or Google might be able to come up with a fully integrated solution someday, each have invested in parts of this. Beyond SOC's, Samsung has their custom silicon which helps them lock down security for their combo touch/display controller. Samsung has also invested a lot into customizing their Knox Secure Folder solutions (and everything else branded "Knox" as well, which is all mostly industry-leading for Android options). Google has the Pixel with their own Titan M2 security chip, and obviously they own the OS.

But it's a lot of work when so much of your engineering is dealing with changes that other companies are making. Google has to keep up with Samsung's hardware changes, because the tail wags the dog there, and Samsung spends a lot of engineering time figuring out how to deal with / customize / fork changes to AndroidOS that Google pushes (while the dog still wags the tail, too). Both have to deal with whatever Qualcomm throws at them for cellular modems, and it required a monumental effort/expense from Apple to only just recently bring up a replacement for Qualcomm's modems.