Because with Brexit on 31st January 2020, the transitional period begins on 1st February 2020 and will last at least until 31st December 2020 (*), during which nothing changes!

(*) Although Boris has put it into law that an extension of the transitional period is not possible, it is highly unlikely that a comprehensive deal can be agreed on during such a short time. The UK government has time until 1st July 2020 to change its position and request an extension of the transitional period for one or two years, so either until the end of 2021 or 2022!

Boris Johnson's Brexit
Boris Johnson’s Brexit