The answer above are right and accurate in my opinion, but wouldn't convince me or help much if I didn't already know. 
Trying to add what seemed missing made me realize that the answer is not as obvious as it seemed to be at first glance.

Why couldn't there be a way to remove the intensive of mining half of the time?

If that was achievable, then the miner would redirect their mining power to another chain that uses the current PoW system, forfeiting the purpose.  However, that doesn't prove that such a scheme can't work or can't exist.  It means, at most, that it would not achieve the intended the result.

I think understating Proof of Work is key to understand how it doesn't make much sense to "pause" mining, just like understanding what animals crossing signs are for helps with the question about how they know where they have to cross, yet without providing an answer.

Analogy:
Mining is voting with energy.  So to make an analogy, attributing a number of vote by unit of energy is akin to using feet or meters.  Changing the unit of measurement will change the number, but the distance remains the same.  Miners will tend to mine with all the energy or power they have.

Misconception:
It may seem at first that blockchain requires mining, which has the side effect of requiring a lot of energy, but it is the other way around.

What is mining?
The purpose of mining is to impose a cost on voting.  For an anonymous vote to mean and be worth something, it must cost and prove something.  Hashing is how that it's done, i.e. we purposely uses electricity, or energy, to give value to votes.  If it didn't cost anything to vote, it would make cheating free, which would makes any votes worthless.  So hashing converts energy into a digital vote.

Each hash proves that an equivalent amount of energy was spent, and that proof is a digital representation of it, aka a vote.  We then chain them so that they pile up, so each link in the chain represent the total amount on energy accumulated in that chain from its conception up to there.

Energy is constantly flowing into the chain, and the chain is backed by the SUM of all the energy of all votes in its entire history,

It also constitute a global clock that provide timestamps.

Blockchain integration:
Integrating data (e.g. transactions) into that timestamp makes it a block, and the resulting energy chain becomes what we know as the Bitcoin blockchain.

So the blockchain contains the ledger, i.e. what we and to set in stone, and each block is one more tick of that clock that seals the latest transactions along with everything that precedes it.  It is an energy chain attesting what happened, at what time, and in which order.

The value and immutability of that blockchain come from the accumulated energy that attest its authenticity, the proof being hashes.

By rewarding honest voters, we incentivize people to spend energy to verify and votes, and the largest decentralized industry is born.

Because honest vote get rewarded, and dishonest votes are very costly, incentives enforce honesty.  The game theory is more complex, but that's the gist of it.

How can we force those voters to stop voting half of the time?
How do we stop that global timestamping machine half the time?

The limiting factors:
Alongside capital to invest in the mining hardware itself, electricity is very often the top limiting factor.

One may have the money and space to put 300x S19 Pro in their basement or in some spare bedroom in an apartment, but can they feed them the 1 Megawatt+ of power required?  I'm sure someone somewhere can find a way, but the electricity requirement is always limiting.  Does that make it unfair and "more centralized" because only those with access to a lot of electricity can mine that much?  I don't know.  Wouldn't they have at least proportionally more mining power if the limitations due to electricity were removed (partially or totally) from the equation?

The foundation of the value of Bitcoin is the electricity it uses, even though all the technology on top of it is required to make it what is is.  The amount of energy itself is the driving factor.  How much mining, how may hashes, etc. are irrelevant, because miners will typically push their capacity to the power they have available.

Comparison with Proof-of-Stake:
If the goal was to do things efficiently, we could pay the cost of centralization.  Proof-of-stake is just a bit in between.  Whoever owns some stake (e.g. token) has a proportional voting power and have no externality.  Stake holders can keep voting in perpetuity without incurring further cost.

If one person buys enough votes, he becomes the king and we are back to what is essentially a centralized system.

In PoS, bootstrapping it requires some method other method of distribution, because there must be some stake before any voting can even happens.

Rewriting the chain is trivial with enough stake or collusion.

In Proof-of-Work, miners buy hardware, which has an upfront cost and maintenance cost, but most importantly running cost.  The hardware is NOT a vote, and does NOT vote either.  It merely converts energy into votes.  As soon as the energy stops flowing, the voting halts.

No bootstraping is required.