This issue was brought up previously in topic #18542, which dealiased the 2 tags. It was argued that the 2 are very distinct and should be separated.
However, leek is currently predominantly filled with images that should be tagged spring_onion, such as Miku holding one. Contrarily, farfetch'd, who is supposed to be holding a leek, is rarely tagged with it. The image used in the original dealias request is also misleading. While it distinguishes between leeks and spring onions, welsh onions are also a type of spring onion and seem to be at least one of the things negi refers to.
Additionally, in art, it is difficult to tell the difference between the two, particularly when they are simplified into the typical depiction of a white stalk with 2 green leaves. Artists then warp the scale and shape of the vegetable, making identifications hard, particularly for inattentive taggers. Many English speakers also refer to the spring onion Miku holds as a leek, which further adds to the confusion. It doesn't make sense to differentiate between the 2 for tagging purposes when only a small portion of posts even bother adding detail beyond the basic Y-shaped vegetable and I can count posts I found from leek that definitively depict a leek on one hand.