There's a part of me that thinks there's perhaps a logical and reasonable explanation, I just can't think of one.

Maybe because the signal strength might not work as users expect?

Signal strength is like the loudness of music being heard. It's possible for music to be quiet but otherwise excellent, or loud but low-quality. However, if it is too quiet, then the "music" becomes almost unintelligible, which the offseted bars should still be able to indicate.

In Wi-Fi, 6GHz and 5GHz are often used instead of 2.4GHz. 2.4GHz would likely win in signal strength. Yet, the others are used anyway, because the others are good for other reasons. However, if range ( ...or compatibility) is critical, then 2.4GHz is used.

Similarly, in cellular, there is a lower frequency e.g. band 8/12/14/17/20/28/71 and a higher frequency e.g. band 1/3/7/30/38/40/41/66/77/78. (Less basically, it can be more granular.)

So this sequence of events is possible: Tower switches the phone to a higher frequency -> speed increases but the signal strength reduces (confusing, but at least doesn't seem bad if there are 3 or 4 bars.) A switch to a lower frequency normally occurs instead if the high frequency signal is weak.

Cellular can be slow due to interference (maybe more common than signal strength issues; the metric to use instead might be SNR/SINR), congestion (maybe more common than signal strength issues; the metric to use instead is confusing, maybe the CFI value (if automatically changed) or RSRQ with a high SNR/SINR might rule it out), the speed of the rest of the network (the metric to use might be RSRQ during a download with a high SNR/SINR), data plan (the metric to use instead might be RSRQ during a download with a high SNR or SINR/QCI (requires interpretation)), and the width of it (the metric to use is BW). So it's confusing, and not exactly that full bars are always better.

For 2G, with each nearby cell (coverage area) basically getting its own channels, signal strength might've been more important, though interference was there somewhat (so there was MAIO planning etc.)

But aside from speed, there's the battery to consider. If the signal strength from the tower to the phone is "satisfactory", it's implied that so is the signal from the phone to the tower, so the phone will have to have an elevated transmit power.

> a logical and reasonable explanation

There is a logical and reasonable explanation. These companies are run by a bunch of sociopathic, unethical people who won't hesitate to lie and cheat if it gets them more money. It's as simple as that.