For the same reason they're not now, and why you currently
don't have batteries and are only just now considering solar panels: cost.
For example, here's the breakdown of the Australian NSW energy regulators supply and demand dashboard: https://aemo.com.au/en/energy-systems/electricity/national-e...
See the scale on the right for demand? The bottom is 6,000 MW. That's 24/7, all year round pretty much. My home state never drops below 6 GW of constant, continuous demand. That's baseload. Doesn't matter what it's made of, doesn't matter what it's components are, if you want to avoid brown outs or blackouts, then at all times there must be at least 6 GW of generation available overnight.
So, applying the 1:3 rule-of-thumb for LiFePO4 power:energy, overnight we have a period of at least 8 hours where we need at least 48GWh of storage - and we're going to use all of it. Of course, that's a number where you scrape through - because to recharge that storage, you're going to have to supply at least double that amount of energy to support the baseload while you do it. So now you need 96 GWh of generating capacity. But solar doesn't have the capacity factor for it remember - 25% at best, over time. So optimistically we'll need to deploy about 384 GW of solar to charge that system. Only...we can't rely on that either, because 25% is...average over time. And we absolutely have to charge those batteries to make it through the following night.
But wait: there's a big mismatch here. We can't just amortize over 384 GW of solar. Because all of that solar might be generating at full power during the day. Or it might be under-performing, or not performing at all. We have this massive surplus we need to have, but our batteries - 48GWh of them - are going to give us maybe 16 GW of power, and likely they'll be able to absorb energy slower then that (i.e. charging would be maybe 90+% efficient). We can't charge them faster then 16 GW: that big array is solely to try and meet an average amount of charge to get us through the next night - provided nothing else goes wrong. And we can't use it efficiently: because we also need the batteries during the day. Clouds over a solar plant kill the output instantly, so the battery has to step in to compensate and retain grid stability.
So the actual amount of battery capacity we need, to get us through one night is going to get considerably larger then 48GWh (16 GW). In fact ideally we actually need...pretty much 384 GW of batteries. Because if our arrays perform well, we need to be able to soak all that power up to have enough charge to get through the night, but we also need enough batteries to sustain the arrays going down during the day and needing to run the grid off the storage momentarily...but we can't afford not to be charging, because on average we're only getting 96 GW - but the lows and highs are very far from that number.
So from that one bit of analysis - and making no accounting for emergencies, equipment failures, efficiency of individual components (i.e. 90% battery charge efficiency + 10% losses in transmission lines etc.) we're currently at a tally of 384 GW of solar, 384 GW of batteries, and we have no redundancy whatsoever in this system. Because we can't get a reliable 6 GW from solar.
Now obviously the picture gets better if you include other things: i.e. wind tends to match solar dips and does work at night, so a combined solar/wind capacity factor is usually about 50%, and with better modelling you could shave some of these absolute margins into more balanced ones, but the problem remains: you've got to charge the batteries, and there's a limited rate you can do it. And it's a problem which gets worse for something like Pumped Hydro, because pumped hydro can have higher energy storage but it has much lower power output as a proportion - meaning it takes longer to charge (it would however be a good backstop for long term storage if we could build enough of it - can we?) There's also some positives - i.e. over time that giant over-sized battery installation is going to get way better cycle life since it's now >1000GWh of storage capacity and we won't actually be using all of it or even a fraction very frequently. We actually have a pretty good buffer over time if we expand the generating capacity further since we could a couple of weeks over no sun without running down our buffer.
Of course...current Australian generating capacity for solar in 2023 - nationally - is 32.9 GW.[1] And globally...there's about 300 GWh of LiFePO4 in existence at all. And the deeper you regularly cycle your batteries, the more expensive per unit they become.[3] Which is a problem because I've just proposed installing ~USD$154 billion dollars of batteries (assuming low-end cost per kWh estimated)[4], more then the entire world supply, to be able to adequately guarantee baseload electrical supply for one state of my relatively small country. Or about USD$25 billion per reliable GW, in batteries alone. Which makes the current expensive nuclear power plants look downright cheap and ITER would still be competitive when it's actually done.
[1] https://www.theguardian.com/environment/2024/jan/04/australi...
[2] https://www.lifepo4-battery.com/News/10-Largest-BATTERY.html
[3] https://gwl-power.tumblr.com/post/130701906811/faq-lifepo4-c...
[4] https://www.nrel.gov/docs/fy21osti/79236.pdf