Because the majority of American adults pay either no income taxes or very close to that.
Which is another way of saying, it's unpalatable to politicians to focus on spending side cuts, in pretty much every regard. Even Republicans have become a party of no spending cuts, they've entirely abandoned the premise (which was a core party pitch for decades).
More realistically, problematically, there's very little spending to cut except for military (which neither party will argue for, save for a few outlier politicians). Nearly all of the above-inflation spending growth is in entitlements, debt interest and military.
The budget deficit and debt interest are high enough now, that they nearly equal (or do equal) the cost of all military spending. So if you want to balance the budget, you'd have to wipe out the US military; or cut military spending in half (shave $300b-$350b), and very considerably raise taxes on the top 1/3.
There's nobody interested in touching the third rail of entitlement cuts. Few are interested in arguing for military spending cuts. There isn't much that can be done about debt interest expenses, US debt per dollar is already historically very cheap (they'll have to go the way of Japan sooner than later, slashing that cost as low as possible with perma low rates, that creep lower with every cycle, reducing economic dynamism and growth along with it, culminating in Japanese style economic stagnation). So in short, all that is left to cut - things that enough people will agree on cutting - is relatively little edge expenses.
That's the reality of why spending isn't looked at more often for cutting.
It's actually still possible to fix this fiscal mess, there's no will for it however, few are willing to eat the shit necessary to do it.
Here's the sane, Save America From Financial Ruin, plan:
1) Slash military spending by $350 billion. Pull back from global policing. Cut almost everywhere as it pertains to the military. This will leave a vacuum for Russia and China, which they will expand into, that has to be accepted. Regional militaries will have to step up to counter that, such as Japan, South Korea, Germany, France, UK, etc. The military is primarily a human employment cost, slashing the military in half would result in two million unemploymend persons, that would have a serious cost initially to it. Over time those persons would become net tax contributors, instead of primarily a net spending cost (soldiers sitting in buildings are epic net cost centers and very poor producers of economic activity and tax contribution).
2) Raise several hundred billion dollars per year by increasing taxes on the top 1/3. Needs to be at least $300 billion, ideally more.
3) Abuse the USD global reserve standard while we have it, and while the dollar is relatively high. Use it to cheaply refinance our debt to as low of an interest cost as we reasonably can. Every $100 billion per year we can shave off of the annual debt interest cost, is a big plus.
4) Freeze entitlement spending increases for ten years. This will involve pain, nobody will be happy about it. It's better than cuts, which is even more ideal. Slightly increase worker contributions to these programs to shore them up further.
5) Freeze nearly all other government spending for ten years, no matter what. It will be painful, that's part of the process. Raise the gasoline taxes by quite a bit, that all goes into infrastructure to offset that that segment isn't going to get any inflation adjustment for ten years.
6) Hammer down costs in education and healthcare, which will free up capital for economic investment and wages. The top 1/4 income bracket receives an income / wealth transfer in the form of the outsized US spending on healthcare. Redistribute that back to the bottom 3/4, by slashing healthcare costs by a lot. This will push up wages for the bottom 3/4, and reduce wages for the top 1/4 (who are the overwhelming beneficiaries of healthcare spending). Doing this right, can both broaden healthcare access/coverage, and offset the hit the bottom 3/4 will take from various freezes on spending increases.
The first five items will get you to a balanced budget, plausibly a modest surplus. After a decade of freeze, relax spending increases to something modest, 2% max, for another decade. You'll be at a budget surplus, the economy will hopefully be larger in real terms, and your ratio of debt to GDP will rapidly begin to decline, as you both outgrow it and modestly pay down some of it. You can also continue to somewhat abuse the USD reserve to have the Fed buy and cancel out the most expensive US debt (effectively debasing some of the wealth held in USD, by slightly debasing the currency, but that wealth is overwhelmingly held by the top 1/3, so it's really a stealth tax on the wealthier block of the population).