Outdoor warning sirens are widely treated as the primary tornado alert. They were designed for a narrower purpose, and the mismatch creates specific failure modes.
The design goal was outdoor audibility
Sirens were built to reach people who are outside and away from other information, in parks, on job sites and in open public spaces.
Coverage is calculated for outdoor sound levels at a given distance, not for what is audible inside a house with the windows closed.
Modern construction, insulation, air conditioning and household noise all attenuate the signal further than the original assumptions allowed for.
Sound does not travel predictably
Wind carries sound downwind and suppresses it upwind, so the audible radius shifts with conditions and is smallest in the direction the storm approaches from.
Terrain, buildings and vegetation absorb and reflect the signal, producing shadowed pockets within an area the coverage map shows as served.
Heavy rain and thunder during the storm add masking noise at precisely the moment the signal most needs to be heard.
Activation policy varies between jurisdictions
Sirens are operated by local governments rather than by the National Weather Service, and each jurisdiction sets its own activation criteria.
Some sound sirens across an entire county for any warning within it, while others activate only in the warned polygon, which produces very different experiences.
Criteria may include severe wind as well as tornadoes in some places, meaning a siren does not carry a single consistent meaning nationally.
The single-channel problem
A siren conveys that something is wrong without saying what, where or for how long, which leaves the listener needing another source immediately.
It also cannot signal an all-clear reliably, since silence is indistinguishable from a failure or from being out of range.
Anyone relying on sirens alone therefore has no way to distinguish a threat that has passed from one still approaching.
Layered warning is the intended design
Emergency managers describe warning reception as requiring multiple independent methods, on the assumption that any single channel will fail for some people.
Weather radio with an alarm, phone alerts and broadcast media each fail differently, which is why the combination is more robust than any one.
The siren keeps a role in that layering as the channel that reaches people outdoors, which is what it was built to do.
Communities that have moved to this framing tend to say so explicitly in public messaging, because the expectation of being woken by a siren is what leaves households unwarned at night.