While I know you specifically asked for the example $SL_2$, I will take a smaller example for now as everything is a bit easier to explain there.
Let $G = \mathbb{C}^{\times}$ be the group of non-zero complex numbers under multiplication. This is an affine variety with coordinate algebra $k[x,x^{-1}]$. The Lie algebra of $G$ is $1$-dimensional, so in particular it is abelian.
Now, all irreducible algebraic representations of $G$ are $1$-dimensional since it is a torus, so such an irrep is given by a homomorphism from $G$ to itself. It is a nice exercise to show that any such homomorphism which is also algebraic is given by $z\mapsto z^n$ for some $n\in\mathbb{Z}$. If we work through the definitions, we see that the representation of the Lie algebra associated to this is the one given by multiplication by $n$ (since the Lie algebra is $1$-dimensional, a representation is given by a scalar).
But it is now also easy to construct representations of the Lie algebra that do not come from any representation of $G$: Just take ones given by multiplication by something which is not an integer.
The above was a very small example to illustrate how things go in general, though everything becomes more tricky when the representations are no longer $1$-dimensional.
A bit more about the general situation: As illustrated, weights for algebraic groups are always integral, whereas this is of course not the case for Lie algebras, so this puts a restriction on which representations of the Lie algebra can come from the algebraic group (when we have some torus that allows us to make sense of weights). Further, representations of algebraic groups have a build-in finiteness condition which implies that the corresponding representations of the Lie algebra are locally finite (i.e. any finite subset is contained in a finite dimensional submodule), so in particular it is not possible to have infinite dimensional irreducible representations, even though these abound when we study semisimple Lie algebras.