The basic answer to why P4 does not stress the ability to modify tables in the data plane is that doing this in general is an expensive complex feature to implement in the packet processing devices with the best price/performance/power ratios, like switch ASICs and NICs.
Yes, it can be implemented in general purpose CPU cores, but such packet processing devices do not ahve the best price/performance/power ratios, but are orders of magnitude worse in those ratios, typically.
The Portable NIC Architecture has in the last couple of years introduced a couple of ways that the data plane can modify tables without the control plane having to do so, that did not exist in previous architectures:
add_on_miss feature, where if you do an apply() operation on a table and it gets a miss, the default_action (the action that is executed when the table gets a miss) can call a new extern function called
add_entry that can add a new entry. It is proposed to be limited to only adding exact match keys, and only the key fields values that were just looked up and missed, but the action and action parameters for this data-plane-added entry can be any of the possible hit actions of the table from the P4 source code.
Admittedly, this is not a fully general mechanism to add new entries, because it does not let the P4 program add any other key other than the one just looked up and missed, nor does it enable adding entries to other tables, only the one that experienced a miss, and it does not support anything except exact match key fields.
However, this does cover a large number of use cases, particularly flow tables where you want to add a new entry if you get a miss, and in particular a new entry that will match future packets with exactly the same key.
This is implemented in many switch ASICs and NICs.
(2) The ability to delete old entries in the data plane that have not been matched in a configurable time interval. See the documentation for pna_idle_timeout = PSA_IdleTimeout_t.AUTO_DELETE in the PNA specification: P4 Portable NIC Architecture (PNA)
This ability is also not a fully general capability to delete arbitrary table entries without control plane help. Again, it is a feature supported by multiple switch ASICs and NICs in the industry, and it goes hand-in-hand with feature (1), usually on the same tables. If you want to add entries at high rate without control plane help, you often also wish to delete entries at high rate without control plane help.