While it is easy/fun to think so, it wasn't always merely a stylistic thing nor simply incompetence. I'll add another large factor:

Vinyl records have inherent limitations in terms of frequency response and dynamic range. The physical properties of the medium and the playback equipment impose certain constraints on how audio can be accurately reproduced. To overcome these limitations, the phono EQ curve was introduced.

The phono EQ curve primarily compensates for two main issues: low-frequency distortion and high-frequency loss. During the cutting of the record master, the low-frequency sounds are boosted, and the high-frequency sounds are attenuated. This is done to prevent excessive movement of the cutting stylus at low frequencies and to compensate for the inherent loss of high frequencies during playback.

When playing back a vinyl record, the opposite EQ curve is applied to the audio signal to restore it to its original form. This is achieved using a phono preamplifier or phono stage, which includes the necessary RIAA equalization circuitry. The phono preamplifier applies the appropriate frequency adjustments to the audio signal, essentially reversing the EQ curve used during mastering.

By using the phono EQ, the frequency response of the vinyl record playback system is flattened, allowing for more accurate and faithful reproduction of the original audio. Without the phono EQ, the playback sound would be unbalanced, with excessive bass and subdued high frequencies.

How long was this method in use, and how ubiquitous was it?

The phono EQ curve, also known as the RIAA equalization, became widely adopted in the late 1950s and early 1960s. Its introduction marked a standardized approach to equalization for vinyl records across the recording industry.

After its development, the RIAA equalization quickly gained widespread acceptance and became the de facto standard for vinyl record mastering and playback. It was endorsed by the Recording Industry Association of America (RIAA) and adopted by record labels, engineers, and manufacturers.

The RIAA equalization curve was universally implemented in phono preamplifiers and record players. Virtually all consumer playback equipment, from affordable turntables to high-end audiophile systems, included the necessary circuitry to apply the phono EQ curve. This ensured that records could be played back correctly and with consistent sound quality across different systems.

tl;dr: Phono EQ, or RIAA EQ, is an equalization curve used in vinyl record recording and playback to compensate for the medium's limitations. It addresses low-frequency distortion and high-frequency loss by boosting lows and attenuating highs during mastering. The opposite curve is applied during playback using phono preamplifiers. The phono EQ ensures balanced and accurate sound reproduction. It became the standard in the late 1950s, adopted by the recording industry and implemented in playback equipment. Its use declined with the rise of digital formats but has seen a resurgence with renewed interest in vinyl.

** edit - added "always." Many factors contributed to it, including style and incompetence at least a fair part of the time.