What do you mean? Private companies can certainly make strategic investments in other private companies to commoditize complements, partner on technology, explore potential M&A and for other reasons too.
See: Coinbase Ventures, Slack's fund, etc.
See: Coinbase Ventures, Slack's fund, etc.
Someone made a similar comment about YC a few weeks ago – how if you were building dev tools, even though $150k for 7% results in a lower valuation than many founders could find elsewhere, you could consider the equity the price for access to the YC network and existing companies to use your product.