Because it is a border that the UK swore would be open as per the 1997 Good Friday agreement.
This, according to the very same BBC:
“What does the Good Friday Agreement say about a hard border?
“A lot less than you might think. The only place in which it alludes to infrastructure at the border is in the section on security.
“During the Troubles there were heavily fortified army barracks, police stations and watchtowers along the border. They were frequently attacked by Republican paramilitaries. Part of the peace deal involved the UK government agreeing to a process of removing those installations in what became known as “demilitarisation”.
“The agreement states that “the development of a peaceful environment... can and should mean a normalisation of security arrangements and practices.” The government committed to “as early a return as possible to normal security arrangements in Northern Ireland, consistent with the level of threat”. That included “the removal of security installations”. That is as far as the text goes.
“There is no explicit commitment to never harden the border, and there is nothing about customs posts or regulatory controls.”
Sounds like no reason a border resembling the pre-9/11 U.S.-Canadian border couldn’t exist. Ireland is an island. You shouldn’t need more of a border beyond customs controls, unlike Ireland is actively undermining border security in ways which would themselves violate the Good Friday accord.