I plug my phone into my 2017 Subaru Outback to play music and charge the battery, but why can't it also act like a giant antennae and increase the cellular signal?
Are external antennae not a thing for cell phones?
Technically, it's possible. There are both active and passive antennas you could use to boost your signal a bit.
But I think the biggest reason your cellphone doesn't have an external antenna jack is that it would directly conflict with the goals and design of a cellular network.
In particular, you do not want handsets with very high transmit power. A transmitter on full power with an oversized antenna and clear line of sight in every direction -- is a major no-no from the network's perspective. In the worst case, it would be visible out to like 50+ miles. Including to transceivers that might be using the same frequencies. It will bleed over into adjacent cells, increasing the SNR of the entire system, which reduces performance and the total number of effective channels available.
Mobile phones, at least some models, used to have an antenna connector to connect an external antenna, and it was quite common to install such antenna in cars.
Passive antennas do not increase transmission power, not least because that is regulated, but does increase the gain.
One reason that is no longer the case is probably that it is simply no longer needed.
But I think the biggest reason your cellphone doesn't have an external antenna jack is that it would directly conflict with the goals and design of a cellular network.
In particular, you do not want handsets with very high transmit power. A transmitter on full power with an oversized antenna and clear line of sight in every direction -- is a major no-no from the network's perspective. In the worst case, it would be visible out to like 50+ miles. Including to transceivers that might be using the same frequencies. It will bleed over into adjacent cells, increasing the SNR of the entire system, which reduces performance and the total number of effective channels available.