Phone makers now advertise support lifetimes in years. That number is a supply-chain commitment as much as a software one, and it is expensive to extend.
The chip vendor supports the platform, not the phone
Much of the low-level software that makes a phone work comes from the processor vendor, including drivers, firmware and the code that boots the device.
Vendors support each chip generation for a defined period, after which they stop producing updated versions for newer operating system releases.
A phone maker wanting to exceed that window must negotiate extended support or maintain the vendor's code itself, both of which cost real engineering.
Security patches and version upgrades are different work
A monthly security patch backports fixes into the version already running, which keeps compatibility risk low and testing scope narrow.
A version upgrade replaces the operating system, and every driver, customization and carrier feature has to be brought forward and re-validated.
Manufacturers therefore often promise more years of security patches than of version upgrades. The two figures describe genuinely different commitments.
Carrier validation adds a queue
Builds destined for carrier-sold devices go through operator acceptance testing before release, covering network behavior, emergency calling and operator applications.
That testing is scheduled against the carrier's own release calendar, which is why the same update can reach unlocked devices earlier.
Each additional carrier variant multiplies the work, so support duration is easier to promise on models with fewer variants.
Older hardware constrains what can ship
New operating system versions assume newer capabilities, and features depending on hardware a device lacks either get disabled or are absent entirely.
Performance also matters. A version that runs poorly on aging silicon generates complaints that a manufacturer may prefer to avoid by not offering it.
Support in a given year therefore does not guarantee feature parity. The device stays secure and current while doing less than a new one.
Regulation is pushing the numbers up
Support periods have lengthened partly through competition and partly through procurement and consumer-protection expectations in several markets.
Because manufacturers build one global platform, commitments made for one region tend to propagate, which raises the baseline for US buyers indirectly.
The practical reading for a buyer is to check when the model was released rather than when it was purchased. The clock starts at launch, not at the sale.