Because, at least in the US, a stray dog must be held for a number of days before it can be disposed of.
The time period varies from place to play but ranges, on average, from 2 to 7 days.
Over thirty states have what are termed "holding period" laws. These laws provide the minimum required period that an animal (usually a dog or cat) must be kept at a pound or public animal shelter before it is sold, adopted out, or euthanized. Typically, the holding period runs from five to seven days. However, it can be as short as 48 to 72 hours in some cases. The holding period allows owners who have lost their pets adequate time to contact the shelter and reclaim their animals.