We can see that in relatively conservative standard Korean, e.g. from the 1996 국어 음성학 by 이호영, as found on Wikimedia, the values of the short 어 and the long 어 are very different: the long 어 is much more central and a bit more closed than the short 어.
Looking at the vowel trapezoid, you can see that short 어 is between cardinal [ʌ] and cardinal [ɑ], and long 어 is much closer to cardinal [ɘ]. Hence the use of the notation /ʌ/ and /ɘː/.
The 1999 entry for Korean in the IPA has the short 어 much closer to cardinal [ʌ] than [ɑ], but the long 어 is still near cardinal [ɘ]. However, for the long 어 that notation used /ʌː/ instead.
There is also a practical reason for phonologists who work with synchronic and diachronic varieties of Korean, as explained here in a footnote from a 2016 introduction to Korean phonology from Young-mee Yu Cho:
However, I follow the general practice of using the symbol /ə/, even though the phonetic studies find this vowel to be quite back and lower-mid, on the basis of a common practice in Korean historical phonology, where the symbol /ʌ/ is reserved for the Arae-A vowel of Middle Korean that was lost in Modern Korean (except in Cheju Island).
The phonology of the Seoul accent of the modern 2020s is different though; with the total loss of phonemic vowel length across the vast majority of speakers, the 어 vowel has ended up closer to cardinal [ʌ] - thus your perception is correct. Figure 1.3 of Jiyoung Shin's chapter from the 2015 tome The Handbook of Korean Linguistics compares these modern Korean vowels specifically to the recordings of cardinal vowels of Daniel Jones from the 1910s.
It is mentioned that the lips are slightly more rounded for modern 어, hence the notation /ʌ̹/ with the rounding diacritic that can sometimes be seen in the literature. I've yet to see anyone quite dare to suggest that 어 has become [ɔ] in modern Seoul Korean, although this is already the case in 평안도 사투리 the Pyeong'an dialect, where there has been an 어 - 오 merger.