During the maker onboarding flow, you have an opportunity to review and change you URL path. Once your profile is live, that URL path is permanent.
The primary reason is Search Engine Optimization (SEO). Search engines (Google, Bing, etc.) are an important driver of traffic to your profile and listings. If you have high quality content that people are searching for, over time your pages will start to rank for certain searches. People will then see your pages in search results and be able to travel to them directly.
Another reason is links from other websites. People that like your content may share links of your URL on social media (Pinterest, Facebook, etc.) or other websites.
All traffic from search engines and other websites is tied to your URL. If you change your URL, you lose all of the associated traffic. We don't want you to accidentally lose potential customers visiting your page, that's why we keep your URL path fixed.