Are external antennae not a thing for cell phones?
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.
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.