You can ask the same for any language, not only PHP:
- Why would anyone want to start something new with Ruby?
- Why would anyone want to start something new with Elixir?
You can choose any reason to pick a language, do you want a language with a lot of demand in the job market, learn Javascript, you will see a lot of frontend open positions, even more React open positions if you want to specialize in something.
Do you want a lively developer community, choose any language, I learned Java, PHP, Javascript and Ruby and all of them always had helpful people and a lot of courses to help me learn and try to code with good practices.
Answering the question seriously (assuming this is not trolling) here are some reasons to start something new with PHP in 2021:
1. Learning PHP 8 and a popular framework like Laravel will make easier to get a job on some small and medium companies. In the last job I had in a consulting firm, customers were asking for more Laravel developers to the point they accepted outsourcing because there was not enough talented Laravel developers in the US to cover the demand. There are thousands of bad PHP and Laravel developers, but there is not enough supply of good ones, so if you are talented and follow best practices, choosing PHP will make you shine between all those average developers.
2. I probably won't develop anything new with PHP 8 this year, but there are a lot of existing libraries and frameworks (Laravel and Symfony for example) that will benefit from implementing the new features of the language (reducing mess or making code more concise and readable). For the end user of those libraries and frameworks, very little will change since the interfaces don't change too much in order to allow backwards compatibility, so the existing ecosystem is a great reason to use PHP.