There's an big argument to be made for the efficient investment of "taxpayer money".
Then again, the SLS program does fund the private space industry. While it's easy to dismiss it as a mere "jobs program", it did / does provide a bed for the next generation of engineers to learn, gain knowledge, become experts and so on. It also provided a bed for contractors and subcontractors to grow their businesses. Just like the Apollo program did.
The big difference with the likes of SpaceX is that the latter aims to provide a completely integrated platform with all the components. At a fraction of the cost. In a way, SpaceX aims to mimic Apple's business model. It's already a commercial brand in it's own right rather then some white-label name.
While choosing the cheaper, efficient option would seemingly be preferable, a public government isn't a private customer. It's the 'res publica' that represents society and its manifold interests. So, that leaves us a poignant question: Would the cheap price and efficiency offered by a single private actor who does everything, really be worth trading in supporting a diverse, competitive, advanced space industry as part of a public mission?
After all, when taxpayers point fingers to the Senate, let's not forget that this institution exists in a democratic model which the people of the United States have chosen as part of their own representation over the past 250 odd years. Pork and barrel politics are - ultimately - part and parcel of a well-functioning consensus model that doesn't let one dominant voice determine whose interests should be favored at the expense of all others.
Finally, in the grand scheme of things, NASA's budget in FY2020 represented 0.48% of all federal expenses. As far as the taxpayer is concerned, the cost of SLS over a decade pertains to a negligible fraction of their contributions through taxation on an individual level.