I do realise that taxi drivers were surveyed extensively. However, it all comes down to the questions they were asked. If they were asked about a single man wearing a sweater with "DIVE" on it, having his wrist bandaged, and travelling from Point Zero (which was close enough to the crime scene), there could be a variety of answers. Apparently, there wasn't a single answer that fit all criteria.
If a taxi driver did not notice a bandaged wrist or if the passenger was wearing a dark trench coat and was calm and polite, this passenger would not attract attention. Let's assume all single men, regardless of their clothes, had to be reported. The police could then receive numerous reports about men catching cabs at different places and going to different locations.
If there were many such reports, how could the TMPD track all those passengers down if they all paid in cash? These days, people use cards, so the police could have transactions to follow. But if you have many reports that partially fit the picture, it leads nowhere.
We don't know if the police got anything potentially interesting, as they only disclose what they think is useful to disclose. Maybe they have no leads and no particularly interesting information about taxi passengers. I do not favour this scenario, but I don't see why it should be dismissed as impossible.
Re: distance, to drive from Setagaya and Shibuya at night takes about 15 min, to walk - 1.5hr for a young athletic man. Although, the closest rout out of Setagaya would be towards the east to Meguro Ward. If you start from Setagaya City Hall, and walk eastwards towards the Meguro Ward boundary, it would take roughly 30-40 min.
I also find that the sweater is a bit conspicuous and would likely be picked up by cameras, even with poor resolution. Now, just an idea, half-serious—what if he wore the sweater inside out, either on purpose or inadvertently, to make the letters invisible? Was it a print or real embroidery? Or maybe he found an old raincoat or a loose trench coat of Mikio's that fit, something Mikio did not wear very often, so the siblings would not remember it. But if he did have a car, in that scenario, it wouldn’t matter what he wore.