That's true, and I'd forgotten it, so it does make you wonder if they were hoping to go some distance.
However, re-reading the Met's press release, I'm not sure it's 100% clear whether the taxis even stopped.
"They then spent slightly more than three hours walking around the Whitechapel Road area. They tried to flag down three black cabs, but were refused.
"They then left the area on foot and were last seen walking along Brick Lane, towards Bethnal Green Road, at 22:01hrs on Saturday, 7 January. (https://news.met.police.uk/news/update-appeal-to-find-missing-couple-and-newborn-baby-460784?utm_source=rss&utm_medium=rss&utm_campaign=Subscription&utm_content=current_news)
It may be that by "being refused", what's meant is that the taxis had their For Hire light on but were seen on cctv failing to respond to being hailed. While that shouldn't happen, in practice it does, of course. But it may mean that their intended destination wasn't that far away - except that the police have no way of investigating because none of the cab drivers stopped and asked (maybe).
Given the conscious (and rather expensive) decision to take a taxi from Essex to East Ham, I'm wondering if they were actually just trying to go back to East Ham, and ended up doing it on foot because no one would stop for them - which begs the question for me of what - or who - is in East Ham.
JMO/Speculation only