Difference between beta reduction and substitution
The $\beta$ reduction is defined by using capture-avoidng substitution. That is,

$$
  (\lambda x. M)N \longrightarrow_\beta M[x \mapsto N]
$$

So it's the definition. Under the $\beta$-equivalence, $(\lambda x. M)N \equiv_\beta M[x \mapsto N]$, but syntactically, $(\lambda x. M)N \neq M[x \mapsto N]$.

Note that substitution is a meta-operation on $\lambda$-calculus terms (normally).
A $\lambda$-calculus term $M$ is whether a variable $x$,
a lambda abstraction $(\lambda x. M')$, or an application $M_1 M_2$.
That's it.
There is no substitution.
Substitution is a map on $\lambda$-calculus terms.
Therefore, for example, a term $({+}~x~y)[x \mapsto z]$ is syntactically equal to $({+}~z~y)$.
There is no reduction.
In contrast to this, $(\lambda x. {+}~x~y)~z$ is not syntactically equal to $({+}~z~y)$, although it can be reduced to $({+}~x~y)$.



Also, $(\lambda x. {+}~x~1)~2$ is different from $(\lambda x. {+}~x~1)[x \mapsto 2]$. The latter is equal to $(\lambda x. {+}~x~1)$.