Upsells are an obstacle to completing your purchase. They're annoying. They are added compexity that has a nominal chance of benefiting the user. In contrast, they are guaranteed to benefit the company, since as a numbers game, you'll end up pitching enough upsells to convert n additional sales.
When considering the impact of adding complexity, imagine a toilet. Or, rather, your access to a toilet. Some toilets are a door knob away. Others require you to hunt down a coin and drop it into a slot before providing access.
Which toilet are you more proud to have in your business?