Because gov't grants are awarded by a committee of people with lots of different opinions, while corporations have only one interest in mind.

Governments are accountable to constituencies and/or electorates. Companies to directors, management, stockholders, and other stakeholders, roughly in that order (where stakeholders == employees, customers, vendors, business partners, community, etc.).

Governments can be investigated by legislators, executive agencies, and courts, as well as subject to document requests (e.g., FOIA in the US), and journalists. Governments themselves don't typically operate as an advertiser in the press, hence, putting purse control over coverage.

Governments are typically more long-lived, and hence have longer-lived planning and concern horizons, than corporations.

It's not perfect, but political power is a check on corporate or financial power, one of several.

In theory, the government should be more transparent.