The current CAT regime is relatively transparent and predictable (e.g. child with a defined threshold, tax on value above that). Moving to a CGT charge on death would instead require all assets to be valued and historic base costs established, which is often impractical in practice.
More fundamentally, it shifts the system from taxing inheritances (value received) to taxing historic gains, which can produce highly uneven outcomes. For example, long-held quoted shares with low base cost would trigger substantial CGT, whereas a principal private residence of equivalent value would (presumably) remain exempt. Two estates of similar value could therefore face very different effective tax rates depending purely on asset composition and holding period.
Valid points on the practicalities of calculating historic gains, however, this system creates its own distortions. CGT relief on death/PPR disposal and substantial tax free exemptions incentivise asset hoarding (particularly those assets with large CGT bills), as this is the path of least tax friction. This has been recognised over the years, particularly in respect of land e.g. Vacant site levy, windfall tax. I think the principle that there should be no tax advantage gained from holding an asset until death is reasonable. That way hoarding is disincentivised and capital can flow to where its utility can be maximised.
It would also create behavioral distortions around timing of disposals and estate planning, and raise liquidity issues where tax becomes payable on death without a corresponding realization event.
This is the rationale that largely underpins agricultural relief also and I fail to understand why it should be the responsibility of the taxpayer to mitigate this risk for the disponer, rather than the disponer/beneficiaries to engage in proper financial planning.