1. The owning class lives off of owning things rather than laboring to create/sustain them
2. If someone makes an investment, they don’t want that investment to lose value because the less reserve capital they have to live on, the closer they are to relying on their own labor
3. Even in the individual cases where some loss acceptable (startups, diversified pools, personal attachment, etc), competition means that investment on average will always be pulled towards firms with a better reputation of higher returns.
4. So the firms that don’t grow fail and the market overall has to grow for similar reasons.

Edit: oh, also the return on squeezing labor diminishes over time, so you can’t just grow perpetually by optimizing labor costs