Most of the time because companies are hiring out of necessity. They either need to deliver more, faster, or are lacking technical leadership / expertise. This means the team is under enough pressure to deliver as is and don't have resources or incentive to onboard a junior.

My advice is to keep learning on your own time, push yourself to learn a valuable skillset e.g. React (as you say you do frontend) and make something with it. For a new hire getting familiar with a project is a given, even a senior needs time to do this. However knowing a language, framework, or skill is your responsibility before applying for a job.

I've applied for jobs that were above my experience level but ended up getting hired anyway while I didn't have enough "years" under my belt I knew the domain and tech stack required for the job. This often meant when applying for a senior job I'd get signed as an intermediate and same with intermediate roles where I was hired as a junior. Once your foot is in the door, if you work hard it will be apparent that you're adding the value of more experienced member than your current job title and from there its an easy promotion discussion.

Just my 2cents. Know your stuff, that's your job and stay humble the promotions and opportunities will come.