First, when we (or any venture firm) makes an A-round investment, we typically reserve another 2-3x of the A-round investment size for participation in future follow-on rounds for that company. So a $5M Series A shows up on our books more like a $20M commitment. The other $15M isn't necessarily always deployed, of course, but we also double down even more strongly in certain cases (either out of opportunity or sometimes necessity) so it balances out.
Second, it's not either/or -- we do venture rounds as small as $3-5M and we do growth rounds as high as $100M. Each fund has a blend of both.
In practice, we don't pay a lot of attention to any of this. When we get a great A round opportunity, we take it. Same with B rounds, and same with later-stage growth rounds.
So I guess in summary, it's not so much about investor time and attention but mostly about managing the risks of the different sectors.
Consumer startups are inherently less predictable and therefore you invest only when a company appears to have found its market and be at least partly on the way to success. Whether that be A round or later is irrelevant.
Whereas the enterprise startups are more predictable, in that if you find a great founding team with a good idea, they are more likely to make a success of it in that sector. Therefore you can cast a wider net, earlier in the lifetimes of the companies, than you would for consumer startups.
Or to summarise the summary: A good, predictable bet is better than an unpredictable one. Or maybe, never look a gift horse in the mouth?
I would just take out "only" from your comment -- there are exceptions everywhere. When we talk about patterns like these, they really are only patterns -- the truth is always in the specific details.