Why button show disabled in browser without any code set in html.erb file?

Question

In proj.html.erb file, I have code like below which has no disable feature, but in browser, it is disabled. I am wondering why? I haven't set it in anywhere, there is also other buttons in the same container that works fine.

In proj.html.erb:

<div class="sub-div" >
    <%= button_to "Download file", download_path(format: :csv ), class: "btn btn-primary btn-sm", id: "download_btn", :method => :get %>
</div>

When check in browser:

<div class="sub-div" >
  <form class="button_to" method="get" action="">
         <input id="download_btn" class="btn btn-primary btn-sm" type="submit" disabled="disabled">
  </form>
</div>