Short answer: 
Supply‑side economics is convinced that investment is not primarily driven by demand‑side stimulus (like welfare to low‑income households) because it assumes that producers invest based on expected long‑run profitability, tax/regulatory conditions, and the cost of capital, not short‑run fluctuations in aggregate demand. This is a theoretical commitment, not an empirical consensus. Corporate Finance Institute Wikipedia

Below is the deeper breakdown you’re really asking for — why supply‑siders believe this, and where the tension lies with multiplier‑driven demand‑side thinking.

🧠 Why Supply‑Side Economics Downplays Demand‑Driven Investment

1. Producer expectations dominate the theory

Supply‑side economics assumes firms invest when marginal tax rates are low, regulation is light, and profitability of future production is high.
It treats investment as a forward‑looking decision: firms expand capacity only when they expect long‑term returns, not because current demand is temporarily boosted by welfare transfers.
Corporate Finance Institute

This is why supply‑siders focus on:

• tax cuts for high‑income earners
• corporate tax reductions
• deregulation
• incentives for capital formation

These are seen as structural drivers of investment.

2. Aggregate demand is treated as secondary

Supply‑side theory explicitly positions itself as the opposite of demand‑side economics.
It argues that increasing aggregate supply — not demand — is the engine of growth.
Wikipedia

In this worldview:

• Demand follows supply.
• More goods at lower prices stimulate consumption.
• Welfare‑driven demand is “consumption‑heavy” and not investment‑inducing.

This is a philosophical stance, not a universal empirical truth.

3. Welfare spending is seen as consumption, not capital formation

Demand‑side economists argue that welfare to low‑income households has high multiplier effects because these households spend nearly all additional income.
But supply‑siders counter that:

• This spending is short‑term.
• It raises consumption, not productive capacity.
• It does not change the structural incentives for firms to invest in new productive assets.

Thus, they classify welfare as redistribution, not growth policy.

4. Crowding‑out logic

Supply‑side theory often assumes that government spending (including welfare) must be financed by:

• higher taxes, or
• higher deficits.

Both are seen as reducing private‑sector investment:

• Higher taxes reduce returns to capital.
• Higher deficits raise interest rates and “crowd out” private borrowing.

This is why supply‑siders argue that welfare spending can reduce investment.

5. Multiplier skepticism

Demand‑side economics relies on the multiplier model, where spending by low‑income households triggers further rounds of spending and production.
books.core-econ.org

Supply‑side economists reject this for two reasons:

1. They believe multipliers are small or temporary.
2. They argue that consumption‑led booms do not create lasting productive capacity.

In their view, only investment‑driven expansions shift long‑run aggregate supply.

🔥 The Core Clash: Short‑Run Demand vs Long‑Run Supply

🎯 So why are supply‑siders convinced?

Because their theory is built on structural incentives, not demand dynamics.
They believe:

• Investment responds to profitability, not consumption.
• Welfare boosts consumption but not productive capacity.
• Long‑run growth requires supply‑side reforms, not redistribution.

Whether this is correct is a separate question — and many economists argue the opposite.

If you want, I can now break down:

• the empirical evidence for and against supply‑side claims,
• why multipliers matter more in low‑income households, or
• how modern economics blends both approaches.