Why can't I add a split environment in a newenvironment?

I wanted a simple environment for making lists of equations, so I made one that includes an equation and a split environment. The code won’t compile and the error seems unrelated. The most pressing thing I noticed was the "You can’t use `end’ in internal vertical mode." The problem is definitely the split environment.
Here is a MWE.

documentclass{article}
usepackage{amsmath}

newenvironment{env}
    {begin{equation}begin{split}}
    {end{split}end{equation}}

begin{document}
begin{env}
    x
end{env}
end{document}

If I remove the split environment from env, everything compiles just fine. Additionally, If the following example compiles just fine. What is the problem then?

documentclass{article}
usepackage{amsmath}

begin{document}
begin{equation}
begin{split}
    x
end{split}
end{equation}
end{document}

Here is the LaTeX error upon running the MWE.

math.tex|11 error| LaTeX Error: begin{split} on input line 9 ended by end{env}.
math.tex|12 error| LaTeX Error: begin{equation} on input line 9 ended by end{document}.
math.tex|12 error| Missing $ inserted.
math.tex|12 error| Missing } inserted.
math.tex|12 error| Missing } inserted.
math.tex|12 error| Missing } inserted.
math.tex|12 error| Missing cr inserted.
math.tex|12 error| Missing { inserted.
math.tex|12 error| Missing { inserted.
math.tex|12 error| Extra }, or forgotten $.
math.tex|12 error| Missing $ inserted.
math.tex|12 error| Missing } inserted.
math.tex|12 error| You can't use `end' in internal vertical mode.
math.tex|12 error| LaTeX Error: begin{equation} on input line 9 ended by end{document}.
math.tex|12 error| Missing } inserted.
math.tex|12 error| Missing $ inserted.
math.tex|12 error| Display math should end with $$.
math.tex|| Emergency stop.
math.tex|| ==> Fatal error occurred, no output PDF file produced!

documentclass{article}
usepackage{amsmath}

%usepackage{xparse} % not needed in current latex

NewDocumentEnvironment{env}{+b}
    {begin{equation}begin{split}#1end{split}end{equation}}
    {}

begin{document}
begin{env}
    x
end{env}
end{document}