The EU has a history of being really slow at negotiating trade deals unfortunately.
Further, the EU enforces VAT fees and implements protectionism with these methods to favour EU-based businesses, even when no EU competitors exist. These fees have a trendency to reduce economic activity within the EU and prevent cheaper alterantives from outside of the EU being effective as they end up becoming a similar prices or more expensive when the customs fees are applied (despite trying to pass on the savings from signficantly lower production/service costs with less gross income than an EU company which results in a similarly priced product that the customer sees).
Even with a worse trade deal than the EU, the UK can stand to benefit because it won't be required to tack expensive fees on top for the purposes of protectionism of industries that may not even be in the UK to begin with.